1 in 3 Under the Pump !, page-186

Nothing to do with where you hang around, you don't read.

Median
The median is the value separating the higher half of a data sample, a population, or a probability distribution, from the lower half. In simple terms, it may be thought of as the "middle" value of a data set.

Mean(Average)
In mathematics and statistics, the arithmetic mean ( /ˌærɪθˈmɛtɪk ˈmiːn/, stress on third syllable of "arithmetic"), or simply the mean or average when the context is clear, is the sum of a collection of numbers divided by the number of numbers in the collection

Take the below numbers.
1 2 3 4 5 7 8 20

The mean (average) of this set is 6.25
The median is 4.5

Big difference, yes?

Now take your example.
The middle wealth portion of this set is say the range of 4-8.
So the mean (average) of the middle section of this is 6. Quite close to the mean of the whole set.
But the median of this set is 6. Wildly different to the median of the entire set.

Maths lesson aside, property prices are reported as median, therefore to compare with wealth you must use median wealth not mean (average).