The C152 has a fixed pitch prop, which means that as the aircraft accelerates, the RPM will go up.
You're probably leveling off at top of climb, checking and setting the RPM, and settling into your IMC scan. Over the next few minutes, the mighty C152 continues to (modestly) accelerate by a knot or two. Result - the RPM has gone up, and you need to retrim slightly. Wait another couple of minutes, and it may have gone up a bit more.
A general problem with fixed pitch aircraft for IMC is that they are power/performance unstable. As they speed up, the engine RPM increases, and it produces more power, thus making it speed up some more. You compensate by reducing throttle. The aircraft slows down, so the RPM decreases, and it produces less power, thus slowing down more. You add power - repeat as necessary
Your instructor knows this of course, and this minor irritating additional workload is what makes the C152 a good IMC trainer
The practical solution is to set power slightly below max RPM, so that when the inevitable lift comes along and you start pushing forwards, the RPM rise stays below the red line. When the inevitable sink comes along later, and you need to pull a bit to maintain altitude, the RPM will reduce and the aircraft will slow down, but hopefully not too much. Providing you responded soon enough, and gently enough, to the first hint of lift or sink, the power will take care of itself. If you find you need to keep adjusting the power to maintain straight and level, it means your scan isn't keeping up, and you are in a form of pilot induced oscillation. Your instructor knows this too
