It's happened to me as well and it is annoying. If in economy I will tend to carefully consider the row I pick, based on time until I fly the sector, what the seat may is showing, my experience of CE loads on that service (if any) etc. and then make a decision. I get the impression (but I'm sure I will be corrected if this is not true
) that if the flight is still some way off the seat map will often zero out quite a few rows of ET that then get freed up closer to the actual date. Generally I'm happy to opt for the furthest row forward available on the seat map and take the gamble, as usually it works out fine.