What is the basic definition of an amp-hour?
1) the battery may not remain at 12 V during the whole discharge (and may start from over 12 V when fully charged). However (depending on the selection of the cut-off voltage (lowest voltage it discharges to), 12 V may be a reasonable value.

2) when discharged very quickly (and 1 hour is quick), most batteries tend to deliver less total charge. A more usual discharge rate (especially for claiming capacity) is C/10 -- a 10 hour discharge. This your battery may be expected to deliver 10 A for 10 h before falling to less than 12 V.