Fascinating topic - I'd love to hear from more folks on this.
I, unfortunately, have little to offer beyond general speculation: the body tends to produce the most intense and profound reactions to rare and novel stimuli, and all subsequent introductions of those stimuli are mediated by that initial response - that is, the body responds differently on the subsequent occasions from the initial reaction, but *not the same as* that initial reaction. Unfortunately, it would require real research (as in, funded research) to accumulate the data needed to simply figure out what questions to ask, much less analyze it all.
So, it doesn't surprise me if one's second experience with a compound is different, even substantially, from the first.
Important to distinguish, I think, between "psychic" results and physiological results. Most logs, it's all about sensations; perfectly natural, and a worthwhile source of data, but that's WHY we get strict about diet, and track sets, reps, poundage & lifts: the bottom line is not 'did I feel the same or better?' but 'at the end of the cycle, did I gain or cut according to my plan? Do I have results consistent with my previous run, or not?'
So yeah, chiming in w/ partyman I'd like to from guys who did X twice or more and hear about how the different experiences went - and also seeing how the actual results of successive runs stack up.