There is a difference between a threshold # and a minimum.
Grots= Minimum. 1-10= 1 boss. 11-20 = 2 boss...
Heavy= Threshold. If you have 10 or more then you can have x. Then if you have 20 or more then you can have 2x
If was the same for space wolves. 750pts = 1 HQ. So in an 800 pt. game you needed 2 HQ.
You are correct on the statement of the difference between a threshold and a minimum. But, the issue is that both unit entries are either both a threshold or both a minimum. With the same wording of "for every 10...", it's highly unlikely for one to be a minimum and the other to be a threshold.
Now, we all know that some of GW's rules can be ambiguous at best, but in this situation, it would've been very simple for them to have worded them differently if they intended one to be the minimum and one to be the threshold. "For every grouping of 10..." or "for every block of 10..." would have been a much better conveyance of a minimum, I believe. That shows that for every grouping of 10, no matter what number they're at, would open up either the option (boyz) or the cumpulsory (runtherds).
What I've seen so far is that people have been focusing on the optional vs. mandatory aspect of the ruling to justify the stance of how it works. Doesn't matter what the rule is after the "for every 10...", because that's the important part. Just because you
must take a runtherd for every 10 gretchin doesn't change the requirement of what number of gretchin is required.
Now, we'd all be very happy for GW to just go ahead and FAQ this and just sort it all out, but we have no idea when it's going to happen. Until then, the way the threshold is worded, "for every 10", it seems, to me at least, that it's either a treshold for both, or a minimum for both.
At least, that's my 2 cents on it.