Many cannons list 2 rates (or a range of rate) of fire in the specs. For example take this tier 10 German cannon:
When installed on a tank the actual rate of fire is shown:
What determines the actual rate of fire?
Edit: Too clarify: I'm looking for the reason that influences the rate of fire shown for the gun when installed on a tank, not of the rate of fire calculation in a battle.
The rate of fire on a vehicle can be influenced by the turret installed on it. In the case of the L7A1 the gun will fire at a rate of 4,29 rounds/minute using the stock turret and it will fire at a rate of 5 rounds/minute using the Porsche prototype turret.
Both the vehicle and the turret and affect the guns RoF. In your case, the gun is only used by one tank, then the turret affects it. Usually, the same gun is used by many tanks and usually gains Rof with higher tier tanks.
For example French 90 mm AC DCA 45 gun is used by ARL V39,AMX AC 1946 and AMX AC 1948 and has reported RoF of 6.67-9.52 r/m. When fitted on ARL V39 (T6) it has 6.67rpm, AC 1946 (T7) it has 8.7rpm and on AC 1948 (T8) it has 9.52rpm.
The 105mm gun on T29 has 4.38rpm on the first T123-turret and 5.26rpm on second T136-turret.
