The point is that it matters very much to a dealership and
they see a significant difference between the two.
Dealer staff aren't usually trying to look at overall expenses--that's for the accountants to bother with--but the profit/loss per vehicle. This is enforced by nearly everyone's income (on the sales and finance side of the business) being determined on a per-sale basis.
Simplistically, each car on the lot represents a loan. The longer it sits there, the higher the fees & interest paid. Dealers want to move each unit as quickly as possible to keep the repayment fees as low as possible, and to prevent eating up dealer holdback, floorplan assistance, and other incentives. Nearly all of these are calculated on a per-car basis. If it did not matter to dealers, they (and their salespeople) would not know how long the cars stay on the lot--because it directly tells them how much money they're losing per day on that vehicle. Moreover, nearly every salesperson and manager at the dealership know exactly which cars are the oldest cars on the lot, in rough (if not exact) order of age.
The various ways dealers make money--holdbacks, incentives, rebates, floorplan assistance, inflated MSRPs, documentation fees, etc.--all exist to offset the expected costs of doing business--floorplans, commissions, taxes, regulatory fees, etc.
When Car A hits the lot on Day N, the dealer is sitting on 100% profitability from the sale (let's just assume 0 haggling & they get sticker price + all their fees, etc.). Each day the car sits on the lot, they drop further from 100%. The dealer and salespeople are highly incentivized to sell for as close to 100% as they can.
Beyond that, a dealer is in a very unhappy position when the cost of having Car B on the lot is high enough that it begins decreasing the profitability of other cars sold. How does occur? In simplest terms: Car B is on the lot long enough that it's actually eaten through all the cash assistance available to the dealer for Car B, and is now eating through the cash available on Car A.
Let's say the dealer loses 1% profit per day on the lot. It takes 100 days to reach 0, obviously. On day 101, the dealer is now at -1% profit, and that has to come from another car on the lot. It's not unheard of for a car to be on the lot for 300-400 days. I've rarely seen much over 400, but it happens (especially with oddly configured models). In our simple, imaginary scenario, that 400-day-old car has eaten through not only its own profit, but that of 3 other cars.
When you arrive on Day N and Car B has already been there for 100 days, the dealer is significantly further from 100%, and is going to be more willing to bend further to move Car B than s/he is to move Car A, which is sitting on 100%, and whose profit the dealer is strongly interested in preserving.
There are further factors often at play here, as well. Say you are a Sales Manager at the dealership. You have a salary, but on top of that, let's suppose you are paid bonuses based not only on number of cars sold (like salespeople), but also on average days-on-the-lot. The better you are at moving your inventory, the more you make. Have a few cars sitting on the lot significantly longer than the others? Well, now your bonus is fucked up.
The longer a car has been sitting on the lot, the higher your possible chances of working out a better deal purchasing that car. Once a car hits a certain age on the lot, the dealers may even dip into their holdback and other assistance dollars to get the thing moved.
Note: I am not attempting to suggest that the dealers are behaving entirely rationally. I'm just pointing out that these things matter to dealers, who typically look at everything on a profitability-per-car basis.
Disclosure: I know enough about bonuses from selling the oldest cars on the lot to feel confident in saying that lot age matters very much to dealers in a number of ways--enough so that they're willing to pay a salesperson more to sell the 400-day-old car than the 20-day-old car. It matters. The 20-day-old car might make you $300. The 400-day-old car could make you $1,000. When John and Jane Doe walk on to the lot to find their new sedan and you have almost exactly what they're looking for in a 400-day-old model and exactly what they're looking for in a 20-day-old model, which one do you suppose you will try to sell them?