It's really hard for me to explain D: so you'll have to bear with me a bit haha
A lot of software I know of doesn't put the tremolo at the end of a long note, or if it does it's at exactly the same place in the note each time, where as if you're playing violin for example you can pop the tremolo anywhere along the note and no two notes will sound exactly the same.
If you're playing an actual violin you can adjust the sound of each note so much but software is so limited, like the strength of each note will feel roughly the same.
There's a certain 'synthetic' sound to software engineered sounds, I can't put my finger on it or explain it very well but even though the software is great (and getting better!), generating a natural sound is something that is not easily done ><.
I hope this made some sense to you!