[Client/Gameplay] CDR Glyphs give more CDR than stated

Flat CDR Glyphs say they give -0.83% CDR. A page with 9 of these Glyphs and no other source of CDR should give 7.47% CDR, and the Client rounds that to 7.5% CDR.

In game, that page gives 13% CDR, or something closer to 1.45% CDR per Glyph.

As much as I like getting more CDR for free/cheap, I'd like to know exactly how much CDR I'm getting from my runes so I don't go overboard buying ones I don't need...