Hmm, I have had multiple bottles from both of those batches and I didn't notice the difference like you did. Maybe something happened to your 4/20's prior to you purchasing them? i.e. UV exposure, hot/cold swings etc.? Or maybe there was a diff and I just didn't detect it.
As far as brewing it, I made my first attempt at it last month and it turned out fantastic. I really rushed the process too in order to get a couple bottles into a local competition - primary 4 days, secondary with first dry hop 6 days, keg with second dry hop 3 days at 30 psi, then bottled for comp. I bottled an extra one to sample on the day I knew the judging would take place. Tasted awesome!!! I have to wait a couple weeks yet for results to be announced. Anyway, the point I'm getting to is that the beer left in the keg that I continued to drink had its hop aroma and some hop flavor fade extremely fast. By week two it tasted quite different. I don't know if it is because of the particular dry hop varieties used or that in combination with dry hopping in the keg (which I never did remove btw). Either way I've never had an IPA fade this fast. Next time I plan to do both rounds of dry hopping in fermentor and not in keg. Maybe 1 round in primary and 1 in secondary. I usually like to dry hop in secondary (I've done both many times) but I also like to keep the transferring to a minimum.
Cheers!