It is 'normal', however, that doesn't necessarily make it right. (or for that matter wrong)
Though I've installed, 1000 + liners, - I can only speak of the 6 or so manufacturers that I've used, but I would imagine they are all pretty similar.
When they make a liner, they cut it a little short, if they didn't you'd have wrinkles, the liner needs to stretch into the corners to fit correctly. How much smaller they make the liner depends on what the expected temp and sunlight will be when it's installed - a warmer day with full sun will make the liner stretch more. The company we use has a 'summer' and a 'winter' cut, the former being (~) cut 6" short and the latter being cut (~)2 -3" short (I'm guestimating the shortage - it's only as an example). Putting a winter cut in in full July heat will cause wrinkles because it has much more stretch in 90 deg weather, oppositely - trying to put a summer cut in on a 50 deg cloudy Oct day might not even work, if you get it in, the corners are going to be 'tight as bowstrings' (it sounds like your neighbor's corners aren't that bad

)
An installer may be able to alleviate some of the tightness in the corners, I'm assuming it's the deep end ones and not the shallow end, by pulling a little extra liner down the side walls into the deep end corners, but if you do too much the tile pattern will no longer be perpendicular to the deck.
In short, it's not a problem, and won't hurt the liner to have a little tightness in the corners above the water - it's the water pressure that keeps it tight to the corners - if ~6" down into the water the liner isn't tight to the corner, you have undo stress on that corner and the liner's life will be shortened by the strain.
I hope I explained this well enough, if you have more questions on it, please ask :-D