foxbat

Today, I tuned a WRX using my LM-1 lambda reader, I tuned AFR at 12.5 at lower RPM, 11s mid RPM dropping to 10.5 just before red line. I removed the O2 sensor & it was perfectly clean, not a spot.
Immediately after finishing wit the WRX I started tuning an Evo 7 using the same LM-1 & sensor, naturally I tuned it with leaner AFR values than the WRX, 12.5 lower RPM, 11.9-11.7 mid RPM dropping to 11.4 by red line, I removed the O2 sensor was covered with black soot what gives??
The sensor is relatively new & calibrated.