Bootstrap 3 does not support IE 7, it supports IE 8 with Respond.js. However IE8 has support for certain CSS properties that IE7 does not handle. The main one that works with the entire Bootstrap system is
box-sizing: border-box
There's an IE7 css and a polyfill for Bootstrap here: https://github.com/coliff/bootstrap-ie7
But even with those things, if you use in your own custom css styles that are not supported by IE7 then they will not show up styled as on modern browsers.
There's other classes like inline-block that don't work with IE 7 either, so in your own CSS you need to use fallbacks.