That's because when you do an oil change, you're meant to fill the oil, then run the engine for a minute or so, wait 5 minutes with the bike vertical and level, and correct the oil level. This accounts for what's normally "somewhere in the engine", in the oil filter etc.
You should always check the oil level with the bike vertical & level.
And a couple of cm below the line sounds like a long way down.
Either way - it doesn't sound like it's losing oil, it just sounds like inconsistency when filling / measuring it.
Adopt a consistent method, and keep checking over a few thousand miles. That'll tell you how much it's actually using.
arrrf.
