Sorry, but stall is not just a function of angle of attack (alpha). It's also a function of the Mach number - even for 'low speed' stalls; it can be affected by the angle-of-attack rate (alpha-dot) and it can be affected by the weight and normal acceleration (total wing loading) due to wing bending changing the actual wing profile.
Civil aircraft with leading edge devices may considerably exceed that 15 degree value - stalling AoAs in the low 20s would not be unreasonable for a slatted aircraft, for example.
The 7% warning margin rule is not an absolute - there are aircraft out there with less "if the stall warning has enough clarity, duration, distinctiveness or similar properties". (The latter being a direct quote from FAR25.207(c) as of Amdt 25-42) 5% might be a typical number for the lower margin aircraft. Not sure which category the 737 falls into.