Glacial Shroud Sometimes gives me 10% extra cdr equalling 20% cdr???

I meant to create this thread a few days ago but I came across it a few games a go. I had a glacial shroud and no other cdr items. I also had the 5% cdr mastery and I had 10% scaling CDR Glyphs. I was around level 15 and some how had 33% CDR If I recall correctly. Eventually I got a and it never increased my CDR% further but with a my cdr was were it was supposed to be but somehow previously while only having 10% scaling Glyphs CDR + 5% mastery CDR I was at 33% cdr when it should have been at 23%

If I was level 18 and I had the same items my cdr was supposed to be 10+10+5=25% CDR. But I wasn't lv 18 though when I noticed I had 10% extra cdr...and no I did not have blue buff. I noticed this bug a few times but never looked into it until I added up my Actual CDR from items/etc and I was puzzled hard.

The items I had when I noticed this.