I have a Trek 750 Multi Track(women's) -- says 21"; purchased in 1990s, occasional rider i.e. several times a year but not weekly/dail but I still love it and am planning a 2+ day longer ride with one of the R-to Trail trips so want it in good shape. Very comfortable etc. When I check tire sizes I seem to have 2 different: Front 700-35 C Back: 700-38-C Spec for bike of 1997 seems to be 700-35. Would there be a reason for the 2 sizes? Thanks.