Is this your own spreadsheet from when you started?

Yes

Are you absolutely certain that the drift/lay bet percentages are calculating correctly? I.E. that firmers are sometimes bet or lay propositions as are drifters.

Yes


EDITED: Actually, upon thinking about it, you could be experiencing a temporary trend.
I thought about what you said a few weeks ago myself about a high priced horse spoiling an otherwise good day. It sometimes certainly works out that way, but to balance it out, when a drifter goes from $3.00 to $5.50 and you can get up to $6.00 on Betfair, it makes up for it.

This is what I thought, it may be trend or a correction, time will tell. (only watching for now)