Start here:
http://en.wikipedia.org/wiki/History_of_PolandThis isn't unusual. I have relatives who referred to themselves as from "Bohemia", which was perfectly legitimate at the time, and someone in 1901 describing themselves as from "Russia" could refer to a lot of places that haven't been part of Russia for many years. Part of a lot of European research is figuring out the history of the place your folks came from, because it influences what sort of records there are and where they're kept.
It can also give you a good idea of why they left. Surges in emigration can be related to wars and famines, changes in government, economic problems at home or economic booms in the destination. One of my relatives, who was closely connected to a French Catholic group, may have been caught up in the furore around the "Associations Bill" of 1901; he emigrated in 1903 to work for them at a mission in Canada. Until I started researching him I had no idea of this little bit of history (
http://trove.nla.gov.au/ndp/del/article/14455617 ).
What turns up on the census might also depend on how the questions were asked. In 1891, the birthplace was asked for, and in the case of a British person born abroad, "British Subject" or "Naturalised British Subject" was supposed to be added.
For 1901, in order to increase the accuracy of recordings of British subjects born abroad versus naturalised British subjects versus foreign subjects, they decided to record nationality for all cases (e.g. writing France, French subject, instead of just France). Perhaps this is why your ancestor answered "Russia/Russian" in 1901 but specified where they were born in 1891. Or perhaps it's simply that someone else answered for him/her in 1901 and wasn't sure of the details.
