That's the nature of MIDI files. They don't encode audio, they encode a set of instruments with a list of notes. It's up to whatever's playing them to provide what those instruments actually sound like.
Here's an example of the same MIDI file being played back on several different devices: https://youtu.be/eiMP-PlL6VM?t=954 It's clearly the same song on all of them, but it sounds slightly different.
Modern browsers do not support MIDI playback on their own, so chiptune.app is presumably doing it on its own with its own (likely intentionally retro) sound font providing the instruments.
Additionally even if macOS had its own system-level MIDI synthesis (not sure if it does), GarageBand would actually probably still be different since it's main purpose isn't MIDI playback -- it's to be a DAW.
Here's an example of the same MIDI file being played back on several different devices: https://youtu.be/eiMP-PlL6VM?t=954 It's clearly the same song on all of them, but it sounds slightly different.
Modern browsers do not support MIDI playback on their own, so chiptune.app is presumably doing it on its own with its own (likely intentionally retro) sound font providing the instruments.
Additionally even if macOS had its own system-level MIDI synthesis (not sure if it does), GarageBand would actually probably still be different since it's main purpose isn't MIDI playback -- it's to be a DAW.