Poor performance with Mieuseloir's Diary ?

Hi,
I bought about 10 stacks of 12 of these tomes yesterday and I could only get about 20 skill points from them. I never got more than 0.2 points per tome, and for the last ~5 stacks or so I was unable to get anything other than 0.1. Is there a reason for this ? My GS skill level was about 170 when I started and I was told my cap at the time was 193, but even with 120+ tomes I couldn't reach that. Are there diminishing returns when you are close to your level's skill cap ? I don't know, but I sure feel like I was "cheated" out of 20k+ sparks... It can't be that much "bad luck"...

In comparison, in the following minutes I took 2 stacks of "Aid for all" tomes (enhancing magic) and got about 10 skill points out of them.

Is it a bug or feature ? :P

There are diminishing returns the closer to your skill level you are. It's something like:
40+ - 0.1 to 0.5
<40 - 0.1 to 0.4
<30 - 0.1 to 0.3
<20 - 0.1 to 0.2
<10 - 0.1 only

I don't think those are the exact boundaries, but it's approximately how it works.

Ah ! that's what I suspected. Thank you for the explanation !