i just check my docs for the 12 version.
apparently there is a diode in series.

its starts off as a 5.5 v diode which
puts it in impact mode that gives you
a reverse effect in thermo activities
as does an avalanche mode diode which
starts at higher voltages.
the diode reverses this effect to balance
the two as a stable voltage. using the
diode adds .7 volts to the 5.5 V impact
mode diode.
in any case , my meters still are able
to read this with no problem in diode
mode.
P.S.
there are voltage reference diodes
that do not use an internal diode in
series and are around 6..6.2 volts.
this how ever does not effect the
circuit its in. etc.