Union type politics aside, leave is defined in the contract between the employee and the employer. The contract may simply state "as in the company handbook" or it may state a particular definition, in which case, this will supersede the handbook.
So, in short, yes, it's almost certainly legally okay unless specific discrimination against a protected group can be shown (Usually gender or age) and to some degree quite common. Just as negotiating different salary terms or other perks is.
In fact, I ended up with more vacation than others due to a mistake made in my contract generation. That's just life.
Given that you have the asked the question, I'm going to assume you're one of these upset managers so I'll give you a nugget of advice:
Comparing your own rewards to others is natural, but getting bitter and upset will not achieve anything. If you feel it must change, either negotiate better rewards for yourself without trying to make comparisons to others (I.e., justifying it on your own merits) or simply move on and negotiate a better package elsewhere.