How do people get simultaneous offers?
In mathematics, the answer is easy: most American math postdocs are selected on about the same schedule.  There are some variations of 4 to 6 weeks, but that still leaves plenty of room for second round offers from an early school to coincide with first round offers from schools that run later.

Further, there's been a tendency for schools that tend to compete over people to try to race each other to making offers, so the number of simultaneous offers ends up being higher than the number of schools would suggest.

(Also, a one week deadline to respond seems a bit short. What I've seen is more like two, with the ability to ask for an extra week or so which is usually granted if there's a chance of another offer.)

(Just for clarity, by most I mean more than half; there's a big early cluster and then additional positions being considered for months.)