Then this guide is wrong, I guess.
That guide doesn't actually state the "capacity" of a quiver - rather, it states how full Dwarves will fill them in Fortress Mode, which is purely a side effect of the logic used by military squads to acquire ammo (pick up a stack of bolts if the quiver contains less than 25, where bolts never occur in stacks larger than 25).
Additionally, the Fort Mode logic changed in 0.42.03 to fix an issue. They now count training bolts separately from combat bolts. The maximum should now be 98 bolts, assuming the last stack is allowed to overflow quiver capacity.
The wiki also mentions a bug related to the use of training and combat bolts, and how marksdwarves don't really use them properly. Is that no longer the case?
Should be fixed now, as mentioned above.
--edit
And that guide linked to above has more than a few problems. Read it as an absolute last resort if playing the game completely normally doesn't work for you.
It starts off OK, but then it comes up with stuff like:
"The first rule of marksdwarf training is you do *not* assign them a normal barracks. If they have that, they will prefer to train as hammerdwarves with their crossbows vs. actually shooting bolts..."
Where on earth does that come from? Some ancient bugged version of the game presumably.
Ordinary marksdwarves train dodging, biting, crossbow recognition and other standard barracks skills in a barracks. They then go to their archery range to do archery practice when regular melee dorfs would normally start sparring (I.e, not often at first, more so at higher skill levels).