I think the likely explanation is:
FTP is just a single data point along your Power Duration Curve (regardless of whether you do the Ramp Test, 20min, etc.). A race, particularly one lasting longer than an hour, and depending on the terrain, may require high power and high fatigue resistance at several points along the PDC, both sub-threshold and supra-threshold. Last year, for example (prior to joining TR), my training focus was on 10kft GFs. So while my FTP dropped 7%, my 6hr power increased by 28%.
If you really want to compare to your power year over year, you should do power profile checks when you do FTP tests (see p.43 of the new edition of Training and Racing with a Power Meter by Allen, Coggan & McGregor). If you use WK04, Time to Exhaustion (TTE), Functional Reserve Capacity (FRC) and Stamina are also important metrics to track (also described in the book and on Training Peaks website).