Nothing unusual about this.
Rules of supply and demand, the market etc...
It is cheaper to start a trip with BA from almost anywhere then it is from LHR.
Speaking of which, why don't you go with BA in WT+.
It will cost you $1,688.10, so you will save over $600.
You will travel in more comfort, and it will earn you a nice amount of miles and TP with BA.
In fact it will earn you 75% of the point you are going to need in order to renew your silver card with them.
If you were buying an S ticket hoping for a last minute upgrade, you could upgrade the WT+ flight to CW with very few miles.