It's true technically that Ubuntu LTS is based on a snapshot of Debian Testing whereas other Ubuntu releases are based on Debian Unstable.
However, this is a simplistic way of looking at it. It mustn't be forgotten that Debian Stable, too, comes originally from Debian Unstable, and Debian Stable is a very polished final product - so the fact that something spent time in Debian Unstable earlier in its life cycle is not an indication of its quality in the final release. After Ubuntu branches off the packages it imports from Debian, Ubuntu and Debian then independently continue the process of fixing and tracking bugs and getting packages ready for release, following their own separate processes and timelines.
In the case of Ubuntu LTS, it branches off from Debian at a later stage in Debian's process: once Debian's branched it off into Debian Testing - implying Ubuntu will benefit from more of Debian's own testing and bug fixing prior to the point at which Ubuntu's process starts. The primary way in which Ubuntu will benefit is reduced effort getting the packages ready for release. Ubuntu has different release goals and requirements to Debian which indicates that an Ubuntu LTS won't necessarily be equivalent to a Debian Stable.
Another thing to consider is the distinction Ubuntu makes between packages it supports officially (eg main/restricted) and packages it provides only "community support" for (eg universe/multiverse). The packages in the latter will be modified relatively little after the import from Debian.
And lastly, there are quite a few packages in Ubuntu that are not sourced from Debian at all, for commercial or licensing reasons or because they are Ubuntu specific.