Using ExpertFlyer to look at fares between RDU and AMS for travel on 3/12 return on 3/15 I see a DL T fare (TLXSL91) that lists $393 round-trip followed by another T (TLWSL91) that lists $443 round-trip.
TLXSL91 DL T Round-Trip 393.00(USD) E 02/10/11
04/12/11
SU / 1M
TLWSL91 DL T Round-Trip 443.00(USD) E 02/10/11
04/12/11
SU / 1M

But when I go to ITA to price it out (flights have T available) I get the following:
Fare (A1): DL RDUAMS TLWSL91 fare (rules) , psgr type ADT $401.50
Fare (A2): DL AMSRDU TLXSL91 fare (rules) , psgr type ADT $396.15

It appears that ExpertFlyer is reading what should be a one-way as a round-trip.

After playing around a little more, if I un-check the validate fares box the fares seem to display correctly:

TLXSL91 DL T Round-Trip 753.00(USD) E 02/10/11
04/12/11
SU / 1M
TLXSL41 DL T Round-Trip 793.00(USD) E 01/14/11
04/12/11
SU / 1M 07
TLWSL91 DL T Round-Trip 803.00(USD) E 02/10/11
04/12/11
SU / 1M