The idle segment assumes a given managed speed flown with idle thrust plus a small amount of thrust. This gives some flexibility to keep the aircraft on the descent path if engine anti-ice is used or if winds vary. This explains THR DES on the FMA.
The thing is, it can still say THR IDLE on the FMA and have 45% N1, it doesn't always show THR DES as the FCTM suggests. In fact now that I think about it, I don't think I've ever seen THR DES displayed on the FMA.
So THR IDLE on the FMA is not necessarily idle thrust, you need to look at the EWD to see what's going on.