Steampunk science and technologies are 'weird', i.e. 'extremely advanced' with regard to the historical Victorian ones. Such difference could not have happened in a few years, at least two generations is more likely. This places the 'time of divergence' from 'our' timeline very early in the 19th C., with Cugnot's
fardier, Jouffroy's
Pyroscaphe , Bushnell's
Turtle, Fulton's
Nautilus and the Mongolfiere had evolved to fully functional machines by Napoleonic times, while the works of Leyde, Franklin, Volta... already had practical applications. Then, the progresses of sciences and technologies being auto-catalytic, the rate of divergence from 'The Real World™' increased constantly. As a 'butterfly effect' everyday life was changed, with deep consequences on the society.
On the other hand
VSF sciences and technologies are far closer to 'historical' ones, meaning that the divergence from 'our' timeline is far more recent (1850?): such recent and limited differences had not yet impacted on the society.