No this doesn't mean "raise the chances of getting bonuses when forging at 100%".
What I do mean is, say, you're about to level up your combuster to level 10. It takes 34 radiant fire crystals per option, with the maximum being 102. Compare this to the leveling up to level 9, with takes 32 radiant fire crystals per level. What I'm suggesting is that you have a higher chance to get a forge prize box when using 102 fire crystals than you would from using 96 fire crystals.
This applies to other bonuses as well. Using 9 fire crystals to level up an item from 1 to 2 will not have as high of a chance to get a double level up or a 25% heat bonus chance as you would using 48 to level up from 6 to 7.
I would say leveling up from 5 to 6 would retain the same values that are currently in place, and for every rank of fire crystal above level 5 would have an increase in 0.5% or 1% multiplied by the "ranking" (ie, low, medium, high) of crystals used.
Overall this would reward players for using 100% at higher values, and say, 70% or 30% at lower levels. Alternatively you could use this system to replace the failure chance outright, but let's not go there quite yet.