Let's take some snapshots.
Ava just shy of two years old. No words. Three consonants and one vowel in her phonemic inventory. Not able to imitate. History of reduced babbling. Lots of red flags for Childhood Apraxia of Speech.
Ava at three. Decent phonetic inventory. Speaking in multi-word sentences. Significantly reduced intelligibility. Exhibiting many age inappropriate phonological processes. Diagnosis would look a lot like a phonological processing disorder with a motor-planning (apraxic) component.
Ava at 3 1/2. Intelligibility is improving. Many phonological processes are resolving - even the ones that are still age appropriate (cluster reduction, stopping of stridents, etc.). It would be difficult to describe Ava's speech problems as a phonological processing disorder at this point. She has numerous speech errors which are resistant to intervention due to the underlying motor planning component (stubborn, stubborn velars).
Same kid. Same neurology. The "diagnosis" looks different at different points. We get so hung up on a label. We need them for insurance and schools. Parents and SLP's tend to like them too. It is more important to understand the characteristics of the speech at that point in time and to have a plan to remediate the current spectrum of problems. Then you have to periodically re-evaluate with an open mind and be responsive to change.
(Typically, you'd never administrate the same instrument three times in five months. In this particular case it was done because of the timing of Ava's IEP. They assessed her at intake, at the end of the school year, and then at the beginning of this school year. The results are not simply an artifact of retesting though. She has indeed made progress in the areas noted. It's pretty amazing actually.)