i stole this from someone lush:

what youre reading are british re-prints of the classic (and not so classic) storylines.

the way these reprints run is that theyre usually comprised of a longer story-arc that might run a few issues and one or two individual, usually unrelated stories (maybe from a different era) sort of like 'healthy filler' - its not essential to the main story of the comic, but they have extra pages to fill.

also, due to the nature of collectors item comics, the chance of you getting a 1965 issue of The Amazing Spiderman is pretty slim, so these collected edition reprints give people a bit of everything - both modern and classic storylines.