Is the COUNTA() Function Buggy?

This is a marvelous program, and it was well worth the wait, but at least one of the functions is a little dicey. When I apply COUNTA() to a named (header) column, it often gives a result that is much higher than the actual number of non-empty cells in the column. For example, when I created a column named Assistant (for Assistant Professor) and entered six faculty member names in that column, COUNTA(Assistant) returned eight rather than six. When I selected the empty cells below the faculty member names and hit Delete, I got the right answer, six. This led me to believe that Numbers was picking up something in the otherwise blank cells and treating them as non-empty. But there's more. If I then enter some text in another cell in Assistant column, skipping some, COUNTA(Assistant) returns nine; that is, it jumps from six to nine. I suspect that COUNTA() is not keeping track of the empty cells correctly and therefore is not updating correctly. Can anyone reproduce this problem?