Atten F2700-C - why 13 MHz as a reference?

[Scratch.HTF]

This frequency counter has an 8051-compatible MCU which contains two 16-bit timer/counters using a reference frequency of 13 MHz which I consider to be unusual for this application.
For a 13 MHz reference, the highest timer/counter modulus-free result ((13 * (10^6)) / 65000 = 200) which is 99.182% of its maximum of (65536 - 1) before overflow, but if we use a more common 10 MHz reference ((10^7) / 62500 = 160), we only get 95.367% of its maximum.
In this case, a result of 200 would be an easier division ratio compared to 160.