The truth: Only in a handful of cases have people stripped of the right to vote by convictions been shown to have knowingly voted illegally. This makes sense, since who would risk getting thrown back in jail for the sake of one extra vote? Less rarely – though not often – people vote without realizing they're ineligible, frequently because they were misinformed by clueless election officials. Sometimes, it's a case of people being convicted after voting; other times, a person convicted of a misdemeanor, which doesn't disqualify them from voting, is listed as a felon. Again, the fault often lies with flawed list matches caused by typos, clerical errors, and the like.
Typical case: In 2000, Florida claimed that 5,643 ineligible persons with convictions actually voted in the general election. Upon investigation, it turned out the data included eligible citizens with misdemeanors, citizens with convictions after their valid vote, and eligible voters whose names and birthdates matched those of convicted felons.