It seems deliberately confusing to me since there is no fundamental difference between voting now and voting on the day of the deadline, but the way it’s discussed and referred to seems to imply that the correct day to vote would be waiting until the last minute instead of voting just getting it out of the way weeks ahead of time.
As somebody else mentioned, historically 99% of voting was done on election day. Opening the polling places earlier than that was the exception, and the terminology has simply stuck as the practice has expanded. Additionally, many jurisdictions have something materially different about voting early, whether different hours, looser location rules (I used to be able to early vote at any polling place in my county, but not now… thanks, Texas), etc., so it’s useful to refer to distinguish them somehow.