Amplitude inconsistency when exporting

It's important to remember, there is no such thing as "proper dynamic range" in this sense. You might have your personal speakers set to a given volume, but everyone else likes to listen at a different volume and might have their own speakers set to their own preferred volume. And in order for this work, it's pretty important that all songs shared publicly - this goes for musescore.com but also applies to YouTube, Spotify, and even radio and CD's - be recorded at a consistent volume. This process is called "normalization" and has been an integral part of all professionally-recorded digital audio since the beginning.

So what happens is, to prevent your piece from being louder or quieter than someone else's piece, everything is always normalized to the same level, so that people listening to your piece don't have to turn their speakers up or down to get it to the volume they like. In other words, if your music were quieter than everyone else's, people would have to turn up their volume just to hear it - not nice. That's why this standard exists - to prevent that.

That said, if you have some unusual special reason to want users to need to adjust their own speaker volume when they listen to your music, you can disable normalization in Edit / Preferences / Export. But not definitely not recommended unless you have advanced understanding of digital audio and have a very specialized application in mind in which the standard normalization would not be appropriate.