Doesn't the lack of women in the countryside in this age bracket cancel out the surplus of women living in the major cities?
No, it doesn't, because of the net loss of marriageable males from Poland as a whole. The ratios of marriageable females to males in rural areas are about equal, whereas in urban areas, marriageable females greatly outnumber marriageable males.
In other words, women from rural areas migrate preferentially to larger cities inside Poland. Men from rural areas migrate to other countries, so there is an overall deficit of marriageable males in Poland overall.
Also, even if there were a surplus of males in rural areas and of females in urban areas, they would not "cancel out", as the extra males and extra females are not in the same place. They would only "cancel out" if marriageable males migrated to the cities or if marriageable females migrated to the countryside.
A last factor is that the proportion of females who feel the need to get married is higher than the proportion of males. Marriage is simply more popular among females than among males. So even if the ratio of females and males of marriageable age were 1:1, there would still be a surplus of women wanting to get married and not being able to find a willing partner.