[Groff] Unexpected tbl behavior

[Blake McBride]

Greetings,

Playing with tbl, I hit something I can't figure out.  I get the same
result with groff and heirloom so it must be me.  My input text looks like
this:

.TS
tab(;);
L L L L L .
AAA;;BBB;;CCC
_;;_;;_
aaa;;bbb;;ccc
.TE


The first column is, unexpectedly, wide, i.e. longer than needed.

The second column is not left justified.  It has padding on the right and
left.

The third column had no padding on the right, but does on the left.

I don't understand the padding, and worse yet, it is inconsistently applied.

I really appreciate help understanding this.

Thanks.

Blake McBride