The Digital displays as pointed out by the people who make the Fridges Are only a Guide, There are many things to take in account,
1) The Fridge sensor is in the bottom in most cases and as the cold Air sinks the Temp at the bottom will swing by a couple of degrees where as the Temp at the Top can vary as much as 5*c OR 9*f That is Normal depending on how high the cooling plates come up the sides.
2) Fridges that have a stainless steel or Aluminum lining that goes from Top to Bottom like Engel, National Luna and Snomaster are much more temperature stable than fridges like ARB or Dometic where the Cold Plates only come up 80% of the side, The Downside of the Full Plate versions is they can not be used as a DuelZone fridge where as the ARB and the Dometic versions can where you can set them to -10 and still have the Dairy area and the top area of the main compartment around 0*c/32*f +/-.
3) With the full plate versions in order to use them as a Duelzone you really need to buy one that has 2 thermostats / and displays with either One or Two Lids, Being Duel Controls they will use more power than using a Single Zone Type fridge like the ARB or Dometic being as a Duel zone fridge,
If you have a limited power supply the ARB's and the previous model Dometics are the way to go and even if you do have a twin Battery setup a Duel Zone/Twin or Single Lid fridge Like the Snomasters and National Luna's are still going to give your batteries a Good workout where as their single versions won't as much but you loose the duel zone option, The other choice is to run two fridges, Where as if you need the extra storage space but only want to run one fridge thenthe ARB60L/63Qt or the ARB78L/82Qt version can save you power as well as give you the best of both worlds using them as Duel Zones using only the power of a single zone,
The temp Variations can be used to your advantage depending on they type of fridge you buy, I know this first hand having bought and tested many New fridges in the past 5 years, All of the above is more important than the name/brand because you have got to live with them for a long while and get it wrong and you will hate it, If any of the above matches your needs then buy the fridge that opperates the way you need it too because all of the above work differantly and the Brand you want may not work the way you need it too, Work out what you want it to do and Go from there.
Hope that helps.