Odd Spider RTC Behaviour?

I’ve been playing around with an external RTC module (SD2305) and decided to compare its performance against the Spider RTC. I was expecting the performance to be superior but what I’m seeing with the Spider RTC is confusing.

My initial “Quick look” comparison showed that after 24 hours the Spider had lose several seconds as has been observed with other mainboards. I wanted to determine the rate of error so set up a quick experiment.

Using the EthernetJ11D module I connected the Spider to the internet and obtain NTP Time. This is used to set both the I2C and Spider RTC. Subsequent iterations obtain NTP Time and compare this with the two RTC’s to obtain the error in ticks. I then take the Mean error of the past 10 comparisons and send it to ThingSpeak.

The surprise here is that the Spider RTC error appears to be stepped with an increase in the error rate over the period of approximately 1hr but fairly stable over 3 hours. What causes this??

ThingSpeak channel can be found here