4th gear isn't in fact a gear it is direct drive from the clutch, through in input shaft, through the input shaft dog ring and synchro ( also know as 4th gear. This connect the input shaft to the main shaft and to the prop shaft. ie no actual gear mesh.

Some RR tuners dont like 4th gear because in 4th 1:1 7000rmp is normally 140 mph. there are some tuners that just dont like to have a car on their rollers doing 140.

You can never take any two RR readings at face value. Was the ambient temp in the RR cubicle hotter or cooler. for ever 1dec you can reduce your input temp you can expect a 1 hp increase. ( more on tuned engines).
Was the IAT's monitored, were they the same on each session. smaller cooling fan could have a large effect on power.

Tyre size has a impact on power, the bigger the tyres the more power is lost. Were the wheels and tyres the same.

What fuel was used and how old was it. Pump fuel can be different from make to make. Esso is 98 ron, BP 97 ron, Shell Vpower 99 ron. Was it the same in the Tank as before.

Operator variable can make a difference in readings. two different people can get two different results. not massively different but when your chasing number the loss of 10 or more hp can feel like the end of the world.

Factor in a bad day and a combination of the above and you could end up disappointed.

You can also do Pbox 60-130's to get an idea of your power.

BTW a rwd 3 door standard with 205bhp should show about 155-165 at the wheels on a roller. Been there and done that 25 years ago.