Was playing a Delve in Sea of Sorrow Level 80 with a start of 2.0, played the 1st room of 1.2 multiplier, should have brought my next room to 2.4, if we are multiplying here. Instead it added the .2 for a room total of 2.2 & so on…
What’s going on? Please explain…
For any “multiplier”, subtract 1 and add it to the running total. So, as you found out, a starting multiplier of 2.0 becomes 2.2 after finishing a room with a bonus of “x1.2”.
No, this is not the clearest way to express the idea. It is, however, what the devs went with.
That’s a euphemism. It’s just plain wrong. 
The correct way to express is would be +0.2 instead of x1.2
Ok… LoL, I thought I was missing something… my math is correct, however as you both explained it’s not what’s actually happening… I see now… Sigh