What I'm wondering is if the number range in column C is doing this because of something excel did, or if it's an error from further upstream, eg, coming out of the datalogger.

If it's upstream, then that means you're doing some data processing, which is totally fine, but not something you want to do by accident.

If it's something excel is doing, then I would prefer to try different processing so we're not introducing unnecessary noise with what should be lossless data handling.

Anyway try something like this in a helper column:

=IF(VALUE(C13)<10, VALUE(C13), C13 / 10^(LEN(ROUND(C13,0))-1))
This should output the number you want, as like an immediate solution to your woes. It will assume anything greater than 10 (you can make the IF term-test like 100 or 1,000 if you want) is screwed up and divide it by its own size-1 to be 1+change.