HSC and Hardware Interrupt

I have an application is about rotary cutter for corrugated paper board. The main component is a measure wheel and a servo motor for rotate roll cutter. I use S7-1200 to implement the rotary cutter function.

When the material speed is below 150M/min, the accurate is ok, +-1 mm, but when speed faster, the error appear. Of course, it is related to the limit of motor ability. But I find a strange thing, it's about the HSC counter and interrupt.

There is a proximity sensor to detect cut per round. That means when cutter roll rotate one round, proximity sensor sense one time. This connect to S7-1200 input and configure hardware interrupt to read motor HSC value. Theoretically, the HSC count of each rotate should be a fix value, for example, the average count is about 24575. but this value floating all the time. When rotate speed increase (maybe 3 Round/Sec), the countdifferencehigher,roughly between 24560~24590.

So I have two question, The first, there might be a interrupt delay and a HSC read delay, how long is it? And is the time delay a fix value?

The second, is the proximity sensor have a unstable detect, I use a M12 proximity sensor with a 8 mm detect range. Maybe the detect range is too long, so the detect point change with different speed.

Please give any suggest if you have similar experience, thanks.