The error is NOT a problem, if the bias is consistent.  Generally,  I've
found that modern temp probes don't necessarily have to be pricey to be
precise.  Also, how did you check calibration?  And, did you pop the  cover?
Some  have an adjustable screw to tweak temp.

We use a NIST traceable, certified, mercury thermometer for  calbration.
Trick is, you need to put the probes in some sort of  constantly stirred bath
to calibrate, or else positional effects will skew  readings.  That is, you
can't just string the probes out side by side on a  table and expect them
to match.

As long as each of the probes is consistent over the temp range  of
interest,  you can mathematically correct.  Of course, there are  cheap probes, and
even cheaper each day from China, so you may have gotten some  bad ones.

I sometimes get folks like Radio Shack to let me pop batteries in, do a
fast check, take the best from the lot.  I just say we're doing  research.
Average person doesn't care about a degree or two.

Now, when you are measuring over a LARGE temp swing - i.e.,  freezing in
winter, blazing hot in summer, and you want probe accuracy over that  range,
it's going  to cost more.  But your winter test shouldn't be  that much of a
problem from variance over the range of expected temps.

Finally, you need to be sure lead wire length for the probes are the same,
and they should all be matching brands, models, etc.  And, be sure that the
 read-out has also equilibrated.  For outdoor testing, we give everything a
 day OUTSIDE to equilibrate before check calibration.  We've even seen the
warmth in the lead wire affect the probe reading.

JErry


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