It's not an oversight, it's by design. A book doesn't have to calculate anything. Suppose on page xxx I have:
a:=2
b:=3
Area:=a*b
So Area=6
Then on page xxx+5 I have:
If a and b now take on the new values
a:=4
b:=5
then Area=20.
You can understand that, but for Mathcad to know that the latter two values should be used it would have to understand the text. The way Mathcad treats it is that Area is a fixed quantity calculated from previously declared values. What you are asking for is that Area is an implicit function. In the example I give above, it would actually have to be both, depending on context that is only clear from the text.
Valery shows an example from Smath, where you can do what you request (although not the example I show). What he doesn't say is that the help (or perhaps a tutorial?) warns you it's a dangerous thing to do, because you can get unexpected results.