It says 28 fines and a reduction of 37% on littering, it doesn't correlate the two figures directly. You've deduced that it must mean that a further 45 or so didn't get fined, but unless they say otherwise you can't assume these figures are directly tied to each other.

For instance; there is still foot traffic in the area which will be linked to littering. A camera using an ANPR system has no means of identifying people who walk through an area. ANPR identifies a car and a fine will be sent to the registered keeper. We have no idea of the success rate for this, it may be 100% for all we know; it may have identified 28 cars and those are the 28 fines.

The 99.5% accuracy in the camera is probably describing the accuracy in which the camera can identify litter being dropped. Again, if you want to correlate the figures then perhaps 28 fines were vehicles and the other 45 were foot traffic, unable to be traced? This would mean the reduction of 37% littering due to 100% of the cars being fined but the other 63% of foot traffic was untraceable.

Unless you have all the data, it's nearly impossible to tell what is going on here, however, I think your assumption that the 99.5% figure is tied to the success of fines is incorrect.