He must know that though. So what could his philosophy be ?
Well this "Jerusalem should be the capital" stuff is always in the background in the US isnt it, didnt their Congress rule that that should be the case a few presidents ago , but put a clause in that the implementation could always be deferred for six months indefinitely.
So, it comes up every six months to be reviewed .Previous presidents have rubber stamped the deferral -- I reckon that Trump has been thwarted on so many of his promises made before election, that he thinks this is one he can appear to be keeping , whilst not actually going ahead with , using the excuse of deferral for one reason or another .
That apparent promise will win him a lot of serious brownie points with the Jewish lobby groups , and he might be hoping he can use it to lure votes from Jewish democrats to support his dwindling Republican vote too.
His move also means it will cause trouble in the middle east, so maybe he wants the US to have a refreshed reason to hang around there interfering . Wars are very good for US economy.
Also ,the US already have a consulate in Jerusalem- if he was serious in going ahead with this , he could easily have transformed that into a temporary Embassy whilst he built his new one. It could all be political hot air.