The results you get from pool.ntp.org is a random selection from 3000 available servers in the network. The pool is constantly monitored, and badly performing systems are removed from the pool when they stop providing reliable time.
Ideally, you would want to use the pool servers from your region, which you can find by drilling down to find the closest selection from from the web site.
I am not sure how Ubuntu handles their NTP server internally, and short of asking them, there is no way to be certain. As a general rule though, you should be using NTP servers that you know to be reliable and are close to you.
In any case, you should always have a couple NTP servers defined, so you can just let the NTP protocol pick the best source of time.