They do heat the object they are fastened too this is good for a degree or two maybe. Radiant heat sources like UTHs do not heat air.

http://www.ebay.ca/itm/Aquarium-Digi...ht_3879wt_1259

I like these ones they are not expensive and a number can be bought and checked against each other. They have twice the spec'd accuracy as accurites (+/-2ºF against +/-4ºF) I would suggest 5+ and then the comparison is likely to find the ones differing from the others.

It is likely that you are simply going to have air temp issues at 70ºF i would suggest raising the room temp to 74-5ºF this will assist a great deal in holding correct temps.