I'll give it to you straight.
Some films are shot in pure widescreen (e.g. on the film you see thick black bars between frames and the widescreen shape on the frame) while others are shot in OPEN MATTE.
What that means is that it is shot in a full 4:3 frame but INTENDED to be shown in widescreen in theaters. This is achieved by placing excessive headroom on the shot that becomes more intimate (but does not crop the actors) when shown in theatres (the backing plate on the projector would have cropped off these excessive parts to create the widescreen image).
When they were then released on video or shown on TV in the 4:3 days, the width would not be compromised but you would get more image top and bottom (and sometimes you would catch a boom mic either top or bottom that would have been cropped off in theaters).
Many filmmakers preferred this as it meant their films would not be pan and scanned when released on TV and video - it was a handy dual format back in those days.
Here are some famous films that were shot this way:
NATIONAL LAMPOON'S VACATION
FULL METAL JACKET
POLICE ACADEMY I-VI
Kubrick and Spielberg in particular were fond of it, and if you compare any old video releases of the above films with their Blu Ray counterparts, you will see the width is the same on both - just different headroom.
Warner Bros for some reason used to leave a lot of their film in full frame - whether this was laziness or just that they didn't undertsand that it was intended to be seen in widescreen I do not know. The Shining and some of the Police Academy movies were amongst the ones not originally re-cropped for DVD for some reason.
The ratio I have described is not to be confused with 4:3 ACADEMY RATIO however, which cannot be cropped top and bottom as it would cut off actor's heads.
this makes sense watching this particular movie right now (the copy I have is 4:3)
for example, I'm at the part where Lassard knocks the guy down the escalator with his golf clubs, and if the shot was any wider it wouldn't make any sense, everything is lined up perfectly in the middle of the screen
so you can tell it was originally shot in 4:3
These 'OPEN MATTE' films were intended to be seen in W/S in cinemas, etc but were shot 4:3 so that you just revealed more frame top and bottom on TV and VHS rather than Pan and Scanning.
The excess image isn't meant to be seen ideally, but is the lesser of 2 evils VS Pan and Scan.
You just get the odd boom mic from time to time!
WB however chose to put a lot of these out 4:3 on DVD for some reason!