Manual folding in vim

Question

When manual folding is active, I fold lines 21-23 in the following code,

19         while (fgets(line, LINESIZE, fp)) {
20                 line[strlen(line)-1] = '\0';
21                 if (regexec(&reg, line, 0, NULL, 0) == REG_NOMATCH) {
22                         return -1*lineno; 
23                 }
24
25
26                 lineno++;
27         }

but after sometime I add some another lines (for example 2 lines) between lines 21-23. But when I press fore example za command, 5 lines are folded/unfolded.

My question is, in manual folding, doed number of folded lines can be changhed?

Thank you