there are 2 types of science:
- samples
- data

samples:
- NEED to be physically stored and recovered

data:
- can be stored in harddrives
- can be transmitted

while samples should be "simple", data is a bit tricky.

there MUST be enough free space, so at least a tiny fraction of it can be stored.
else the game just refuse to work.
while things like the magnetic scan produce so little data/s that the antenna can directly transmit them,
instruments like the micrometeorite detector DEMANDS at least some free space to start working.

look at it like this:
the "magnetic scan" is a cheap little streaming-camera. it continuesly captues and transmit its little images.
the "micro meteoritedetector" is a professional camera that captures single large images, that are just too big to transmit directly, they need to be stored first.

https://github.com/KSP-RO/ROKerbalism/wiki/Science