They don’t transmit “last” because they don’t have that information. I believe this is documented somewhere, but I don’t recall where.

?

What you see as “last” in TWS is actually the bid/ask midpoint. If you watch very intently, you can see that “last” changes every time the bid or ask changes, and it takes values ending in .3 or .8 if the spread is a half-tick multiple.

?

Richard