htaccess SEF URLS dont work with the word 'drills'

Question

I have a strange problem where my SEF URLs just wont work for the word 'drills' I have just got around the issue by using a different word but Id like to know why this doesn't work.

This is my entire htaccess file:

   RewriteEngine on
   RewriteBase /
   RewriteCond %{REQUEST_FILENAME} !-f
   RewriteCond %{REQUEST_FILENAME} !-d
   RewriteRule ^page/([0-9])/?$ /index.php?page=$1 [QSA,NC,L]

This works perfectly and redirects domain.com/page/1 to the correct domain.com/index.php?page=1

However, if I change the htaccess file to this:

   RewriteEngine on
   RewriteBase /
   RewriteCond %{REQUEST_FILENAME} !-f
   RewriteCond %{REQUEST_FILENAME} !-d
   RewriteRule ^drills/([0-9])/?$ /index.php?drill=$1 [QSA,NC,L]

I just get a url not found error.

To find out where it was going wrong I edited the htaccess 1 letter at a time so page became drills and it all worked correctly until I changed the initial letter. So this works:

   RewriteRule ^prills/([0-9])/?$ /index.php?drill=$1 [QSA,NC,L]

but this is a URL not found:

   RewriteRule ^drills/([0-9])/?$ /index.php?drill=$1 [QSA,NC,L]

Is there something special about the word 'drills' that means I can't use it for SEF URLs?

To fix it I've just used training-drills instead but it's bugging me that I couldn't get it to work with just the word 'drills'