There are several considerations to keep in mind here, of which I'll list a few:
1. young age. Very young children (which, of course, is what the WPPSI is for) are notoriously inconsistent in their testability, which is one of the reasons testing at this age is not considered stable. Even those who are more predictable in their test behavior are also going through rapid growth surges in different aspects of their development at different times, which makes it more difficult to tell when a score reflects the timing of that aspect of development, and when it reflects some kind of stable ability profile.
2. test-retest effects. The recommended retest interval is at least 24 months between readministrations of the same measures. If your DC was tested at age 4-? and again at age 5-10, then there was definitely not a 24 month wait period between the two. Typically, of course, one expects score inflation as a result of retest effects, but children react differently to the experience, and some of them (especially those who like novelty) may put forth less effort when the task is perceived as repetitious. So scores could go down too.
3. relative age standing in the norms. Even if we assume that #1 and 2 were not factors, the first administration was very low in the norms, which means that an examinee does not have to do quite as much on certain subtests to score really well (there's also lots of ceiling, so the structure of the test is less likely to cap performance). The second administration was at an age that is pretty much in the middle, or even slightly toward the upper end of the age norms, so floor effects are less likely.
4. reliability coefficients. Subtests are not the most reliable measure. Psychometrics are more robust at the composite/index level, so one should try to take subtest-level interpretation with the appropriate amount of weight (a bit, but not too much).
5. examiner/examinee effects. And finally, sometimes it's just the rappport the examiner has with the child, or the skill they have for maintaining focus or eliciting full responses. Or the day of testing, and how much sleep or water or exercise (etc.) the child has been getting.