Adjective
overoptimistic (comparative more overoptimistic, superlative most overoptimistic)
Excessively optimistic.
By early September these hopes were seen as overoptimistic, citation and in late October the company cancelled its plans, announcing that exploratory drilling had identified nothing of commercial value. Source: Internet
This bias is providing lower values than it should, so it is an overoptimistic (assuming that low numbers is what one wishes) bias reducing the usability of the measurement rather than improving it. Source: Internet