I don't think you can conclude too much from a 3-5 min test time. There are too many short term variables in heart rate with exercise. For example, I have seen times when my HR at a high cadence is quite high initially and then seems to drift back as my body adapts (at the same power output).

The best way to determine your optimum cadence (ie the cadence that gives the highest functional threshold power output) is to do a 20 min FTP test (or better yet, 60 min test) at different cadences preferrably with a powermeter. It is possible though that your optimal cadence is lower than you suspected.