I'm taking it from RAW here, though; where common sense is concerned I agree with you, Ollieb, but how often do 40k and common sense mix?
It is mostly because of the wording "For every ten" appearing in both the weapon upgrades for Boyz and the slaver upgrade for grotz, and the fact that nobody is arguing that 11 Boyz entitles you to a second Big Shoota.
Why should 11 grotz be more difficult to manage than 10 grotz, or 20 grotz more difficult than 19? Nowhere are we presented with information as to how difficult it is for a slaver to field x number of grotz; that's purely speculation. It could be that in the real world, 19 grotz ARE just as easy to handle as 10, and there's no real way to argue for or against that position from the rules.
Nowhere else does the wording "for every ten" imply "for every ten or part thereof," in all of the examples I can think of [chaos units and ork units are unfortunately all that come to mind], "for every ten" means a 1:10 ratio at minimum.
If I can get 2 Big Shootas only once I have 20 boyz, I can [and must] get two slavers only once I have 20 grotz. It totally is the word "must" that's getting people hung up; if it was strictly optional nobody ever would have mentioned it. Consider the following substitution examples:
For every 10 Boyz you may take 1 Big Shoota to
For every 10 Grotz you may take 1 Slaver to
For every 10 Grotz you must take 1 Slaver.
Again, not from common sense, but from RAW. All three of those examples share the same convention; only the first and third actually exist.