In the past few months, it seems that SunPass has started to more aggressively match the license plate with the transponder and charging the plate rather than the transponder.
This might be due to the fact that in Miami Dade county they have done away with cash tolls and gone to SunPASS/PlatePASS only. Thus, the system is taking a photo of the plate and charging the plate if the SunPASS either is not present or does not register.
Dan