Yep, I noticed this with myself too when I first did some grading a few months ago. There was also the factor that the ones I graded initially did not make certain mistakes or answered in expected ways, such that when I did encounter unexpected answers/mistakes, I had to go back and rethink the grading on the papers I had graded previously. Eg if someone answered in a way that made me think an answer I considered inc…
Grading papers in submission order just introduces a different bias, though.
(For what it's worth, I'm in the same boat and I do the same, because I don't trust my ability to give the papers any true random sorting by hand, so I take the very weak randomization that the submission order gives me.)