The difference is, only Springfield Armory can call their rifle
SAI got their start by making Garands by using GI parts on new-made receivers.
Those were sold,
non que suprise, as "M1"
The supply of Garand parts dried up and they pivoted to building M-14 copies.
They sold those as "M1A." The owner of the company has said that the resemblance of "A" to "4" was also part of it.
It also resembled US military nomenclature, too.
Aircraft use just a single letter to designate different models of the same airframe, so, the F-16 becomes the F-16A, then the F-16B, and so on
By long-standing tradition, Ordnance starts their increments at "A1." So, the M-1911 became the M-1911A1. The M-5 light Tank became the M-5A1, the M-1 Abrams has been the M1A1, the M1A2, the M1A3, and so on.
More trivia: The M-1 Carbine had an M-1A1 version, with folding stock. The M-2 Carbine was select fire, the M-3 Carbine had fittings of an IR scope. When the shortened M-16 was turned out as an official Carbine, it was, naturally, the M4.