As MashMonster said, a digital refractometer is great for monitoring mash progress. It doesn't have to be accurate, to show when sugar level has stopped going up.
My cheapo one, usually gives a brix reading, that converts to an OG that matches my hydrometer to within a couple of points.
I've noticed that, for a consistant reading, the lid needs to be closed. You have to leave the sample to reach device/room temperature before taking a measurement. Any evaporation from the tiny sample while cooling (it's very thin with a large surface area), will increase the density.
If the the lid is left open, particularly with high gravity solutions, consecutive readings can keep going up (even after plenty of time for the temperature to stabilise).
I don't much trust it to determine FG. As the SG conversion, which requires both initial and final Brix readings, doesn't match hydrometer well. But still, it's handy for seeing when ferment gravity has stopped falling.
The difference might be because the Brix to SG conversion factor, depends on what type of sugar you're measuring. And the mix, towards end of ferment, will contain few simple sugars.
Other factor could be any effect, from the presence of dissolved CO2. But that also affects hydrometers, unless you degass before taking readings (dissolved CO2 increases the density of water or beer).
Interesting article
https://wiki.anton-paar.com/en/carbon-dioxide-in-beverages/ but it doesnt cover effect of CO2 on density.
Don't get a refractometer that directly displays 'SG' readings, unless only interested in pre ferment readings.
Even with a super accurate/expensive refractometer, an accurate Brix reading will only convert to an accurate SG, for a single (known) type of sugar.
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