Compression value difference between dry and wet test

[SalvadorP]

What's the acceptable variation between compression values between dry and wet tests?

This is not actually concerning my volvo, but I couldn't find the answer online and I feel being a general question it is as valid for our cars as for any other.

I came back to Portugal and found out my mum had been driving my Polo IV low on engine oil, don't know for how long. The oil light came on and off and I checked the level and it was just bellow minimum. I have done a compression test with good test results (220psi), but since the engine was low on oil I was more interested in knowing if the piston rings were bad.
Wet test showed an increase of compression of up to 16% (260psi).

Is 15% increase an acceptable value?
Also, my intuition tells me if I'm puting oil on top of a piston, I'm efefctively reducing the size of the chamber and therefore increasing the compression. I'm wondering if that could justify the 15% increase or at least some of those percentile points!? But maybe I'm being dumb and this makes no sense.

Again, sorry for making a post that it's not volvo centered, but I wanna do a wet test on my 940 too, so this information will come handy.