In our SharePoint 2013 environment, we had mounted the SharePoint 2010 Content Databases and the sites are running in 2010 mode. As the sites are gradually being upgraded to 2013 Look and Feel, we are working on the custom code upgrade.
My basic understanding was that a site running in 2010 mode in SharePoint 2013 won't be able to work with solutions deployed with "Compatibility Level 15".
But, recently we came across a scenario where we have a site running in 2010 mode scheduled for an upgrade and we had both the 2010 code (Compatibility Level 14) and the upgraded code (Compatibility Level 15) deployed in the farm. We noticed that, for the site, the changes made in the ".ascx" files in the "14 Hive" were not reflected on the site whereas when the same change was applied to the files in "15 Hive", The changes were visible.
Please explain why this would happen?