Htaccess - Rewrite engine (reverse engineering a line of code) - .htaccess

Question

On a site I'm working on, if you enter the url, plus 1 directory, the htaccess adds a trailing slash.
So, this: http://www.mysite.com/shirts
Becomes this: http://www.mysite.com/shirts/
The htaccess that runs the site is quite long and complex, so it's not easy to find or test which rule is causing the rewrite. I was able to track down the issue to this line of code (I think):
RewriteRule (.*) http://www.mysite.com/$1 [R=301,L]
Does this rule match the behavior I'm describing above? It seems to be the cause, but it doesn't make logical sense to me. I don't unsderstand where the trailing slash is coming from.
Can someone shed some light on this for me? Thanks in advance.
Edit: MORE:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^mysite\.com$
RewriteRule (.*) http://www.mysite.com/$1 [R=301,L]

Answer 1

By default apache will add the ending /, you will have to use:
DirectorySlash Off
To disable that behavior which is caused by mod_dir, you can read more about it here.
However if you're trying to remove the / to fix images not showing. That is not the right way to do it, you should instead use the HTML base tag, for example:
<BASE href="http://www.yourdomain.com/">
Read more here about it.
Your current rule as you have updated on your question:
RewriteCond %{HTTP_HOST} ^mysite\.com$
RewriteRule (.*) http://www.mysite.com/$1 [R=301,L]
Means:
if domain on the URL is only mysite.com
redirect current URL to domain with www.
So an example of it would be, if you access:
http://domain.com/blog/some_blog_article
It will redirect the user to:
http://www.domain.com/blog/some_blog_article
Note how it retains everything and only add the www. to the domain.
If you really want to redirect it regardless here is one way to do it:
Options +FollowSymLinks -MultiViews
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^mysite\.com$ [NC]
RewriteRule (.*) http://www.mysite.com/$1 [R=301,L]
# check if it is a directory
RewriteCond %{REQUEST_FILENAME} -d
# check if the ending `/` is missing and redirect with slash
RewriteRule ^(.*[^/])$ /$1/ [R=301,L]
# if file or directory does not exist
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
# and we still want to append the `/` at the end
RewriteRule ^(.*[^/])$ /$1/ [R=301,L]