Is the propagation always bilateral ?

Having done, quite a few "single band" contests along the past years, I had noticed more than once, that at band opening /closure, a few DX stations could be heard at a decent level, even at strong level, but that it was impossible to get through. Until a few days ago, my opinion was that it was because the band being wide open for them, they had stronger QRM level.
I must say that the following graph changed my opinion... at least I have a serious doubt...
Above is the plot made from WSPR 30m spots between Joe K1JT and my station during the 2 first weeks of April. As usual SNR's have been adjusted to take into account the TX-power variations (all SNR's being normalized as if both stations were using 1W).
I had noticed Joe spotting me quite frequently in the morning, while I could not hear him at all. Thought that it could be explained by a lower noise floor at his location. Plotting the above, introduced a doubt, especialy this 2h30 offset between the maximums... (the max being moreless equal). If you have an opinion about this interesting question, or if you want to repeat the same kind of plot with another pair of stations (the data is available for everybody in the wsprnet.org database) you are welcome !