My first bits in this forum.. A great question indeed.
Maturity or Capability of any process is subjective to the needs of the organization / work / task. It is a common knowledge that maturity is measured across multiple Process areas while capability is measured with respect to a single area. When we say a Process is mature, we mean the work carried out involving relevant Process areas are optimal and desirable outcomes are guaranteed. But the needs of the organization may change or sometimes technological influences can alter certain Process areas making them redundant or sub optimal thereby enforcing 'a matured Process until now' to undergo further changes. So, it all boils down to the time when an assessment was made and if there are influences from then on that mandate revision of the 'matured Process'. Process improvement per say, is and will always be continuous and synonymous to an endless journey (of course, in the right direction !).