The USB Standard is 5v +/- .25v. So you would expect all of those chargers to be ok since the phones will be able to tolerate the potential +/- .25v.
As to why they are different, I don't know. Some kind of power optimisation for the particular device, I should think.
As for amps, then they are never a cause for worry. If there's not enough amps then it either won't charge or will charge very slowly.
If you think there's too many amps, there isn't. Amps are effectively drawn not pushed, so a device will only 'pull' as many as it needs. The amp capacity shown is the maximum that can be 'pulled'.
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Apple Chargers new
Apple Chargers new