In the lore of the low pressure Bouler is written down, that it can produce 20 mB Steam per Tick, but this is the value of the high pressure boiler... Is it only a typo?

If you're talking about the FTB wiki they have it backwards yes.

No, in game the lore of the Item says 20 instead of 10

Indeed.

Now, is it only the lore which has an typo, or does it also produce the false amount?

Just Lore

Oh, OK, thank you :D