$wp_query meta_key naming issue [closed]

What I am attempting to do is sort posts by custom fields.

I have a few custom fields that don’t query properly and I get the 404 page. The fields that don’t work I used hyphens in the naming convention, i.e. var-name. When I switch the hyphens to underscores it works, i.e. var_name.

Am I overlooking something simple in PHP or WordPress that causes conflicts when I do this?

• I know I could simply just switch everything to underscores but I like to know the root of issues so I can avoid them in the future.

I am using pre_get_posts to modify the query being executed:

add_action( 'pre_get_posts', 'my_modify_query' );
function my_modify_query( $wp_query ) {

    if ( is_category() ) {

        // Works - underscore
        if ( isset( $_REQUEST['var_name'] ) ) {
            $meta_query[] = array( 'key' => 'var_name', 'value' => $_REQUEST['var_name'] );
        }

        // Sometimes works - hyphen
        if ( isset( $_REQUEST['var-name'] ) ) {
            $meta_query[] = array( 'key' => 'var-name', 'value' => $_REQUEST['var-name'] );
        }

        if ( ! empty( $meta_query ) ) {
            $wp_query->set( 'meta_query', $meta_query );
        }

    }
}

Have additional custom fields I check for, the above example is for brevity and clarification and is the same for additional fields.

EDIT 1

Here are the queries being run by WordPress:

The unsuccessful hyphen

SELECT SQL_CALC_FOUND_ROWS  wp_xxxxx_posts.ID 
FROM wp_xxxxx_posts 
WHERE 1=1 
AND 0 = 1 
AND wp_xxxxx_posts.post_type IN ('post') 
AND (
    wp_xxxxx_posts.post_status="publish" 
    OR 
    wp_xxxxx_posts.post_author = 1 
    AND 
    wp_xxxxx_posts.post_status="private") 
GROUP BY wp_xxxxx_posts.ID 
ORDER BY wp_xxxxx_posts.post_date DESC 
LIMIT 0, 10

The successful underscore

SELECT SQL_CALC_FOUND_ROWS wp_xxxxx_posts.ID 
FROM wp_xxxxx_posts 
INNER JOIN wp_j9L3vi_postmeta ON (wp_xxxxx_posts.ID = wp_xxxxx_postmeta.post_id) 
WHERE 1=1 
AND wp_xxxxx_posts.post_type IN ('post', 'custom_post_type' ) 
AND (
    wp_xxxxx_posts.post_status="publish" 
    OR 
    wp_xxxxx_posts.post_author = 1 AND wp_xxxxx_posts.post_status="private"
) 
AND ( (
        wp_xxxxx_postmeta.meta_key = 'var_name' 
        AND CAST(wp_xxxxx_postmeta.meta_value AS CHAR) = '116'
) ) 
GROUP BY wp_xxxxx_posts.ID 
ORDER BY wp_xxxxx_posts.post_date DESC 
LIMIT 0, 10

If I go into phpMyAdmin and simply switch out var_name in the above query to var-name it works.

Edit 2

I am passing the custom fields as so:

http://mysite.com/?cat=example&var-name=100