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Why are post mount spacers labelled front and rear? And why are there options for 180mm, 200mm etc? And why do some space the caliper out backwards and others shift them upwards?
Surely a single 20mm spacer would do for all?
Rear mounts are 20mm shorter so a 200mm front mount would correspond to a 180mm rear.
The rest of it can be explained by the usual lack of standards in that some mounts are placed in a standard position (Hayes Post & IS) and others are just made up to use the same mounts but stuck in a stupid place (Boxxer, Fox 40, Marz Monster T).
It actually depends.
Your fork may be 160mm as standard - so you bolt up a caliper straight to the fork with no spacers and it takes a 160mm disc.
Your fork may be 180mm as standard - so you bolt up a caliper straight to the fork with no spacers and it takes a 180mm disc.
This applies to your frame too.
So a 20mm spacer, could work *exactly* the same front and rear - depending on the frame or fork.
For example, my Evil Offering is a 180mm rear post mount. My MRP Ribbon fork is also a 180mm post mount. If I want to run 200mm rotors front and rear, I would use the exact same 20mm pount mount adapter.
So...
Surely a single 20mm spacer would do for all?
Is actually true, given you know what your fork and frame mounts are starting at.
Yes I have here a spacer that is specifically front 160mm post mount to 180mm post mount, and another that is specifically front 180 to 200mm, they look different and space out the caliper in different directions, but essentially both are just 20mm spacers so I don't understand why they are labelled so specifically as they could be used anywhere.
Various "base" mounting standards, historical standards, caliper clearance (some brake calipers will hit some other manufactures 20mm spacers)
Also bike brake mounts are not radial, so the caliper angle/mount should match so the caliper is positioned correctly on the disc (different manufactures use slightly different specs, that mostly interchange). Different shapes/designs of mounts can do the same job of putting the caliper in a suitable place (does not have to be the same place, just that the pads are at the correct distance/angle)
Peter Verdone has lots of info on some aspects of it if you curious, as usual if you can skim over the drama there are interesting bits
http://www.peterverdone.com/disc-brake-mounting-systems/
The rest of it can be explained by the usual lack of standards in that some mounts are placed in a standard position (Hayes Post & IS) and others are just made up to use the same mounts but stuck in a stupid place (Boxxer, Fox 40, Marz Monster T).
SquirrelKing's last part was right, and answers your question. It's just design.
There was also a time when forks were 160mm minimum, and frames 140mm minimum - which created this weird 20mm difference. I'm sure someone will be able to speak more authoritatively on that though! But it's non-standard standards.
And... legacy! IS mounts were front and rear specific, so I suspect that carried over into Post Mount adapters so the general public weren't confused about which adapter to buy.
A few people have tried to move to the radial approach, Hope the most recent on their full bikes at the rear, but it's never caught on. Makes perfect sense as you just add the corresponding mm height mount to how much larger (or smaller) disc you're fitting.
But this is the bike industry so simple standards are not tolerated.
As said, just think of them as '+20mm adaptors' so will add 20mm to what your frame/fork are set at.
Just count yourself lucky they're bloody labelled!