"You do not receive any upgrade to your bonus, yes. But you receive the ability to get to the next level, which provides you with an upgrade to your bonus."
Level 12 is the next level.
"Basically, a boost to your army's attack and defense is considered valuable and unlike some Great Building bonuses that go up with each level, here it goes up with every other level."
A level is when your armies attack and defense goes up,on this building. If it doesn't go up...it doesn't level.
"The two levels are still in place because if they're combined together you would need double the forge points to level up the GB. If level 11 and 12 are merged together, you would need 1000ish forge points instead of two instances of 500ish forge points to reach that 1% bonus. So even though there is no increase in the bonus, it works in your favor that the levels are broken down as they are. More players may be willing to contribute fps due to the contribution rewards if the amount of fps needed are lower."
If it costs too many forge points to get to the next level (the 1% increase) adding a level in between levels that doesn't level up your GB is not a level. I don't know what it is...but it's definitely not a level.
Let me ask you this...Is there any other GB that goes from one level to the next without actually leveling up, in some way? Any GB any level?