Cooldown reduction display bug

I was playing ARAM with Casseopia and purchased the following build path >> >> then SOLD while having the new CDR Mastery (Intelligence) and 8% CDR in Glyphs. At this point in my build I should have had 43% CDR although it displayed as 45% CDR

I attempted this build path again in a solo game and found that I could build the item path mentioned above and sell my and still have 43% CDR which indicates that is potentially giving 30% CDR when combined or at least it's displaying as such.

The match in question