They probably annualized the amount for tax purposes...stupid payroll companies. I had a bonus check once that was withheld at a higher bracket because it was higher than my normal check. Annualized, it was as if I was making $half a mill.
little reported factoid - they steal from the poor military also --- but you will never hear about it. A full 20-25% right off the top of any re-enlistment bonus.
And then the damned congress taxes "moving" as taxable income since the military usually does the moving.