This is not a surprise. Above 1200°C most of aluminum hydroxides transforms to alpha-alumina which is the high temperature stable phase.
However below 1100°C a variety of phases occurs. The type and the structure of the phase appearing under heat treatment below 1100°C depends strongly on the initial aluminum hydroxide precursor.
There are 5 crystalline aluminum hydroxides (depending of the synthesis route):

• Gibbsite
• Bayerite
• Nord-Strandite
• Diaspore
• Boehmite

With exception of Diaspore which by dehydroxylation leads directly to alpha-alumina (>800) all other hydroxides give you a number of crystalline phase in respect to the temperature

i.e
Gibbsite crystallize to Chi-alumina (300-600°C) then to kappa-alumina(750°C-950°C) and finally to alpha
For boehmite which is the most common phase by sol-gel you can have successively various crystalline forms from eta-alumina to gamma, delta, theta and finally alpha.

Hope this can help