Not just at Plaistow. At my local station, Totteridge and Whetstone,
the indicators purport to show the time and route (ie Bank or Charing
Cross) of the next 3 trains. Trouble is they're almost always wrong.
You can generally double the time to the next arrival, and the routing
information changes every couple of minutes. As only about one train
in 4 goes via CX in the morning peak, it is especially frustrating to
wait for an indicated train, only to have it turn up via Bank.

I wrote to TfL about this last month, and got a 'we don't care' reply.
They said:-

"These problems will be solved by the new signalling system to be
installed on the Northern line. The systems take time to design,
manufacture and install, but should be in place by 2011."

Four more years to cure a simple problem, of their own making! How
difficult can it be to connect the indicators to the signals? Still,
as we only pay £1,328 a year for the privilege, who are we to complain?