How stable should the temperature should my fridge and freezer keep?

I bought a couple of cheap indoor/outdoor electronic thermometers mainly to check how my heating thermostat was doing and to compare accuracy vs my existing analogue ones, as well as get a warning of just how horrible it was outside each day ... however I noticed they had quite wide temperature ranges, plus min/max recording, and so thought I'd also check out how my fridge-freezer was doing. Put the main unit of one into the fridge, and its remote sensor (on a very thin, non-seal-disturbing wire) into the freezer, both quite central in each case.

Thanks to that I've now found that the temperature inside varies a lot more than I thought. The fridge compartment isn't so bad, varying 2.5 - 6.5'C (36.5 - 44'F) and mainly being 3 - 5'C (37.5 - 41'F) any time I've checked it directly, which is acceptable given it should hover around 4'C / 39'F and most guidelines say it only needs to be below 8 (46)... plus I sometimes leave the door open more than I should and the ambient temperature is quite variable (usually 15~20'C (60~70'F) but could have been anywhere within 10~25'C (50~80'F) on the days in question thanks to a malfunctioning heater) so it's allowed a little bit of fluctuation.

The freezer however shows a minimum of -23'C and a max of a bare -6'C (-9 / +21'F), despite only being opened much less frequently and not for anywhere near as long - and the warmest points are when I haven't been present, as direct observation has only been in the -19 to -13 range (-2 / +9'F). The recommendation and desired setpoint is -18'C (0'F)...

Is this at all normal? It seems to be out of the recommended ranges and it does seem to ice up excessively and suffer from unusual freezer-burn sometimes, especially for ice cream. Sadly these cheapie thermometers only do min/max rather than any kind of data logging or graphing, and these figures were collected over a full week, but I could either reset them once a day or even borrow a datalogger from a teacher friend to get a better picture of how long it spends at each extreme... Though it having more than a 15'C / fully 30'F fluctuation in any case doesn't sound ever so good.

The fridge isn't ancient, and is 4-star rated for the freezer compartment, but it is a hand-me-down of uncertain age (the previous residents didn't take it when they moved out, which is probably some kind of warning) and seems to consume more than it's fair share of power when I connect it through an energy-monitor plug - approximately twice that of my mother's not particularly cutting-edge separate fridge and (larger volume) freezer put together.