I have been thinking the same thing. I can't figure out anything else than maybe her fans (got to admit that myself too) relied too much that the overall votes wouldn't be so high this year now that Sasshi and Mayu are gone. Also maybe people thought that former Sayanee-voters would've given their vote/s to Miru. Oh and hako-oshis too. There we're so many surprise names like NakoMiku, with their votes +50,000 which is higher than Miru's total last year so that came as shocker to all Miru-fans too. So as a conclusion I think that fans thought they wouldn't have to vote that much to get Miru into Senbatsu.