rc_time function has the jitters

I built an application using a joy stick and capacitor and wanted to measure how long it took to charge the capacitor.

The rc_time function does that for you but I found that it wavers somewhat up and down and doesn't stick to a couple of values.

I found if I use an MCP3202 Analog to Digital converter however the values only waver by about +-1.

Why is that? Is there some timing problem with the rc_time function?

Mike