Because two months were added to the original ten (July and August after Julius Caesar and Augustus Caesar) which threw the sequence out.

They would if the year started in March. But it starts in January because of symbolic reasons related to the politics of the Roman Empire. Most of the world eventually agreed and it stuck, but it took a lot of Protestant huffs and puffs.

The original Roman year had 10 named months Martius (March), Aprilis (April), Maius (May), Junius (June), Quintilis (July), Sextilis (August), September, October, November and December.

At that time the prefixes made sense (Quint. = 5, Sext. = 6, Sept. = 7, Oct. = 8, Nov. = 9 and Dec. = 10). The winter period wasn't given a name because not much happened agriculturally during that time.

The winter period was later divided into the months of January and February around 700 BCE and were considered part of the start of the year which shifted everything.

Quintilis later became July (after Julius Caesar) and Sextilla became August (after Augustus Caesar). So if that hadn't happened we would have had a couple more weirdly named months in addition to the ones that seem out of place today.

(Contrary to what some people seem to have learned, it was January and February that caused the shift, not July and August.)

The prefix has nothing to do with the month number they are merely short versions of the names.

FYI there only used to be 10 months in the year, two were added by the Romans - July to honour Julius Caesar and March to celebrate the god Mars (I may be wrong on the second one).