off course - p2atc offset compared to X-Plane

[bootsie 0]

I'm flying the following route:

SBGR IG EGDUP SUMRA ROMIB DORLU KEVUN ILTIT NEPEV CIDER 27S030W 29S020W 30S010W 30S000E 30S010E UVGOD NIBEX FAOR

but out over the ocean my position on the P2ATC map is off against the flight plan track by about 15NM (south of the planned route).  The FMC in X-Plane (FF 767) shows I'm perfectly on track.

Any ideas why this is?  I've bodged it by setting the Allowed Course Deviation to 20NM, but obviously this isn't a fix.

[bjwoody61 3]

I ran into this awhile back with FSX. I did some research and found that all the maps do not agree. FSX was located perfectly with Google earth but to put the same coordinates in Google Maps and I came up with a different location. I some instances, it can be substantial. Its usually the longitude that is off the most. Your fix is about all you can do to solve the off course calls.