Downmixing on Logic/reaper produces distortion; not so in MP

[JonesH]

Hello,
This is slightly unrelated to this specific forum but could be of interest anyway.
I've made some surround recordings and when mixing I usually make a downmix matrix (all in PT HD) as well. However, one track that was mixed did not feature a downmix and so it was done afterwards on the finished 5.0 stem. For this I used Logic with normal downmix settings (L/R to L/R at relatively 0dB, Ls/Rs to L/R at -3, C to L&R at -3) and scaled the levels back so to avoid digital clipping. however, even when the fader gain was as low as -8 dB on the L/R tracks audible clipping still occured both in Logic 8 and Reaper. However, neither DAW measured clipping and the file still had headroom of some dB.

Enter Pro Tools MP; all faders at -3 dB and voila, it all sounds nice and clean. Just what is going on? Please.