With Transformers and Godzilla you're basically seeing the result of marketing and brand recognition. You can hype almost any movie to a #1 opening weekend if it's got a recognizable brand name to build from. If the movie drops like a rock afterward, that usually means it wasn't all that great - or at least not of interest outside the core market. So the wider audience wasn't interested and repeat business wasn't very strong.
See also: Fault in our Stars, Pacific Rim, etc. Opened huge, then fell off a cliff. Core audience showed up, but nobody else. Compare to the first Hunger Games: built-in audience to get started with, but turned out to be entertaining to the rest of the world as well. Good buzz, good reviews, so - opened big, stayed big.
Other side of the equation - poor lil' Edge of Tomorrow. Hyped like crazy, but no brand name. And the Tom Cruise brand is sorta shaky in the US. So, opened below #1... and then just hung in week after week, staying around #3-5, gradually creeping toward 100 mil (which it STILL hasn't hit, but will in another week or so). That's repeat biz and good word of mouth at work.
Unfortunately, modern studio deals are set up so they get the biggest take from the opening weekend, and less of a percentage afterward. So on the books, Godzilla still probably looks pretty good and worth making another half dozen just like it. But Edge of Tomorrow? Meh.
I think the wild card of the summer is Guardians of the Galaxy which will certainly do okay, but might be a monster. Marketing, check. Brand name, sorta check (Marvel, although this property hasn't been done before). But speaking as someone who definitely does not say "oh hooray another goddam Marvel movie", I gotta admit this one looks pretty fun, and I'm okay to go. If Guardians is as nifty as the trailers suggest, it could be this year's Avengers, and for the same reasons.