The corporate tax system is so complex it seems impossible to actually figure out what that 5% means though. e.g. They could be highly concentrated in industries where tax accounting tricks are too hard to do effectively.
Corp income tax means the tax (30%) on profit(income-expenses) retained (not withdrawn to shareholders) year over year. Berkshire hathaway is famous for not paying dividends and keeping profits and never selling shares, so this makes sense. Most companies withdraw or reinvest as much profit as possible to reduce this tax.
The 5% is a relative measure against all other corporations in the USA.