most c1 errors are statistical and not always repeatable from one read session to another.
What does this mean?
Because the reading of a cd-r is not totally deterministic down to a submillimeter accuracy, the reader will sometimes get a wrong reading off a disc and report an error. At another time, when reading the same spot, it may not get the error.
Also, differences between cd-r media and drives can cause differences in C1/C2 results, although from what I've seen, these tend to be small differences, unless the drive has serious problems reading that type of disc.
Further, the speed of reading (when testing for c1/c2) is also a contributing factor - especially on discs which are somehow sub-optimal for the testing drive in question.
The professional testing machine manufacturers and testers recommend that testing be done at a minum speed (1x or 4x usually shows not that big statistical difference between them, but 52x compared to them will).
This due to the simple fact that the higher reading speed one uses for c1/c2 testing, the more the test is actually stressing the servo/optics capability of the reading drive and not the media. To put it simply, the faster the reading speed, the higher the amount of errors detected (on any drive I've seen used for testing) and more of those errors are not actual errors on the disc.
Now back to your specific question: why did PlexTools and KProbe report the same profile of c1/c2 errors, but PlexTools reported more of them?
Possible answers (I'm now guessing without knowing more of the situation at your end):
1) You tested at a higher speed when using the Plextor drive
2) Plextor drive has more difficulty in reading discs error free at high speeds using that particular media you tested
3) There are actual differences between how Kprobe and PlexToolsPro command the drive to report c1/c2 error data (I think this is unlikely, but not knowing the specifics, I don't think it's impossible)
Could you tell us a bit more, by testing the same disc on both drives/software using 4x reading speed (I suppose both drives still support this speed in hardware) and report back the results. Also, if you have the time, use another disc from a completely different manufacturer and give us the low speed test results for those as well.