Probe size (& mfg) and temperature differences between them

[smitrock63]

I purchased and installed a new thermocouple inside the bean-mass area of my IR-7. It's physically only a inch or two away from the one that's in the machine (not sure if original or it's diameter). The funny thing is that when I start roasting the drop temperature reads about the same on either probe, but as the roast progresses there becomes a very large difference in readings (I'm talking 25 degrees more or less). So, not sure about this and wanted to see if anyone has seen this before? The new probe I purchased was from Phidgets (PT1000-4 wire RTD 11cm with 3.2 mm probe dia).

I would suspect the new probe is more accurate, but the wide temperature range bothers me. Any suggestions or advice?

Thank you,