Why does deamination in the lagging strand lead to an increase in the relative number of guanine and thymine to cytosine and adenine?
I think you are referring to this sentence from your wikipedia link:


  There is a richness of guanine over cytosine and thymine over adenine in the leading strand and vice versa for the lagging strand.


Note that this is not saying (G+T) > (C+A). It is instead saying (G)>(C) and (T)>(A). I think that those are not the same as saying (G+T) > (C+A), though that is also true, probably? 

So I think that there is no contradiction there- it is just, as you say, more Ts (relative to expected equality with As) and fewer Cs (relative to expected equality with Gs). 

If I understand correctly, the importance of the GC skew, and the reason that it is interesting, is that it acts as a deviation from Chargaff's parity rules (as explained in the article). These rules state that for any given hunk of DNA, (G)~(C) and (T)~(A), which is the expected composition for (for example) double-stranded DNA, because of base-pairing. 

Edit: for more information on Chargaff's rules, see this other question which I have (through pure coincidence) answered here. Note that the second parity "rule" is a statistical phenomenon, rather than a logically required rule. The wikipedia page on Chargaff's rules is also informative. 

Hope that helps.