As explained by the Tampa Bay Times:
Handwriting experts say no two signatures from one person are the same. It’s why Florida election officials for years have used all the signatures at their disposal — sometimes more than a dozen — when they authenticate a voter’s signature on a mail-in ballot.DeSantis wants to rein-in that long-standing practice. Vote-by-mail signatures “must match the most recent signature on file” with the state Department of Elections, DeSantis declared in February. A bill moving through the Florida Senate would make that one-to-one match the law.Some election officials say limiting signature samples could make it harder to authenticate the identities of those who vote by mail, perhaps leading to more rejected ballots. Signatures change over time, they say, and are often affected by the choice of pen, the writing surface, fatigue or a person’s health.DeSantis’ own John Hancock has undergone a transformation during his time in government, as demonstrated by 16 of his signatures compiled by the Tampa Bay Times from publicly available sources between 2008 and now...The most recent signature for many voters may be the one they used when they signed their driver’s licenses at a Florida Department of Highway Safety and Motor Vehicles office. That signature is often recorded on a digital pad with a stylus — not with a pen, like how a ballot is signed.
My most recent signature "on file" with the state is the one on my driver's license, which is only two years old - but looks quite different from my current signature.