Charging amps should change very little due to the battery SOC, until the regulator starts slowing it down at the SOC close to 'full'.
The variation in amps is probably more related to clouds, humidity, sun angle etc.
Check data sheets for a similar factory made panel.
Example from the 75w Photowatt panels I am wiring up right now:
With 0v (short circuit) amps is 4.7
At peak power (17v) amps is 4.4
Open V (0 amps) is 21.6
If the 12v battery is even at 6 volts, mabe it gets 4.5 amps. If it is at 16v, you get 4.7 amps. That's only about 5%.
G-