Errors in levels of subtable in R

Question

here is my problem : I have a table with 3 columns and 12 raws that looks like this :

when I execute : data$exp, I get : h h h h h h U U U U U U

Levels: h U (what I expect).

But When I execute cordata[1:6,]$exp

I get: h h h h h;

Levels: h U.

I don't understand why I still got U in the levels since I took only the first 6 raws.

Any help would be greatly appreciated

Thanks!