Dunno, I'd say it happens more often than not. For us they use a combination of DPD, Hermes, Yodel and Royal Mail/Parcel Force. We have Amazon Prime and live in a fairly out of the way location so it's probably a combination of those two factors.

Amazon has sent a lot of Prime packages to me via DPD, but it seems to depend on when I order it. If I order on Saturday, I have a feeling they have very little choice as to which courier to use and as such it tends to be DPD.

I really prefer DPD because you can track the vehicle and you also get a delivery slot. Recently, Amazon have delivered some stuff for me via Royal Mail and ParcelForce (I ordered in the week) and whilst the packages did get to my house the next day, the tracking was much less informative/useful than what was offered by DPD.