FMS Calculated time to waypoints discrepancy vs. GPS

[rjb4000]

Posting as requested! The discrepancy seems to occur any time a lower flight level is selected as cruise altitude. I descended from FL380 to FL300 and the time to destination jumped up by about 60 minutes. Every leg shows a longer time value than the TTG value on the PFD (and reality matches the TTG value). 

Full flight plan: 

OEJN MIGDA L677 DARAX/N0465F360 L677 CVO L617 NOZ/N0453F380 L617 TANSA UL613 AMAXI TRL UM601 RUTOM/N0455F300 M601 QUENN Q123 USEPI/N0459F290 Z924 SUPUX/N0468F310 UZ924 RIPAL UM603 GERVU/N0364F130 R59 ALT LEMI

The first screenshot is between DORAS and RONAB along M601 at FL300. Winds ~280/71 at the time of the image. The second screenshot shows the difference in ETA between FL300 and FL310 selected.