I wish there was an easy way to find when those allotments were created but I've had mostly bad luck looking that kind of thing up. The FCC can really make it hard to find things sometimes.
The way I understand it is that when the move to Atlanta was granted for WHMA, it created the 100.7 allocation near Anniston, which also allowed WLXY in Tuscaloosa to upgrade and move from 100.7 to 100.5 closer to Birmingham.
I think that WZLM in Dadeville (which was on 97.3) was coerced into moving to 100.3 to let 97.3 in Birmingham upgrade. Then, as WGZZ it later moved to 94.3 as part of the chain that allowed WAOQ to move from Brantley to Goshen and play to the Troy market. That squeezed out the full spacing for 100.3 near Auburn, which is why it had to be applied for by Marble City Media as a 73.215 exception thing.
If this is correct, it means little WZLM/WGZZ has been a cog in the wheel of THREE different frequency shuffles in Alabama: 100.5 moving to Atlanta, 97.3 in Birmingham and 100.3 in Troy.