There are no good sources on this. All best guesses are that TEC's membership have dropped hugely and they're making up the numbers. I never formally renounced membership when I left over a quarter-century ago (maybe I should have, but several years away without a request for a letter dimissory should be grounds for removal) so I wouldn't be surprised if I were still on their rolls.
But it's odd -- the chart you posted seems now to have disappeared.