Quick answer: Ensure the Baro Pressure is set to STD or 29.92 / 1012 hpa after you have passed the transitional altitude
Oh wow in my early days I was screaming so hard at the screen this exact thing. So smug I was that I thought I knew all about flight level and altitude. I was missing one key thing…. I hadn’t altered the Baro pressure.
Baro Pressure is not just about ensuring you don’t crash into the ground because you were out by a few hundred feet, it’s also about ensuring all planes past a certain height all follow the same altitude. If everyone had different baro pressures, my 18,000 feet would be different to someone else’s 18,000 feet. Having everyone set their pressure to the same number means air traffic control know exactly at what altitude everyone is, and so do you. The Baro Pressure standard number is 29.92 inch or 1012 hpa
So you start off from the runway at the Baro pressure set through ATIS, and also reported from the Tower, then once you go past your transition altitude (different depending on your country but on the FMC it says what your transition altitude is. However if you listen to the wording of air traffic control this lets you know which Baro pressure you should be at.
Please climb 10,000 feet, Climb 12,000 feet, clim 15,000 feet all indicate you are working on the Baro pressure set from Tower and ATIS. Once the wording changes to Flight Level 200 (20,000 feet) or flight level 250 (25,000) this indicates your Baro Pressure should now be set to standard (you can pull the Baro Knob and it Should change the display to STD or change it manually to 29.92 Inch or 1012 hpa.
The two images below demonstrate the difference in readouts. The top one shows 35,000 feet at 30.00 inches of pressure, while the bottom one shows FL 350 at STD pressure (29.92)
This should resolve the issue and avoid you crashing into other planes or shouting at ATC.
Happy Flying.