Read this section of the Wikipedia article about significant figures. When there are trailing zeros and no decimal point, the number of significant figures has to be specified explicitly. Thus, 1050 may have 3 or 4 s.f., and 1000 may have 1 to 4 s.f. The result of rounding of 1050 to 2 s.f. is 1000 to 2 s.f.
Edit: Rather, the result of rounding of 1050 to 2 s.f. is 1100 to 2 s.f. if the round half up tie-breaking rule is used.