Say the bonus is $8000 and $6800 are set for a 401k (pre-tax) deduction. Here's the tax breakdown. Does this look right? Shouldn't the taxes be calculated on the $1200? ($8000 minus $6800 due to pre-tax 401k contribution)
Social security and medicare taxes are still levied on 401k contributions at a total of 7.65%. Looking at your table, ($496 + $116)/$8,000 = 7.65%.
Federal taxes were 22% of $1,200 (0.22 x $1200 = $264) which is inline with IRS rules, as pointed out by users in the comments. State taxes look like they are levied on the $1,200 as you expected. But, you can look up the corresponding rates and check the numbers to be sure.