Tracks 'mastered' to same loudness level sound different loudness-wise

Mastering (properly) is a VERY technical / complex process.  Plugins and stuff make it easy to do with presets and stuff like that, but there are a lot of things that could factor into that.  As an example, you could have some low frequencies in one song, that are triggering the limiter (but aren't very audible, because they're either outside of the range of human hearing, or just not very noticeable) whereas, if you have higher frequencies (midrange frequencies, etc...) that are causing the limiter to trigger, then they can cause pain to your ears, because they are RIGHT THERE, hitting your ear sort of at "bullseye" or whatever.  (if that makes any sense). 
 
So even though in the end result, you might have two waveforms for the mastered tracks that look similar, if you look at them through some sort of spectral analyzer or something, you'll PROBABLY see that the "louder" tracks have more action happening in the midrange frequencies, where the quieter tracks might have more mud & stuff, happening down in the sub-bass frequencies where you can't hear them.
 
There are other things that could factor into that, but that's one thing I would suggest checking (if you haven't already).

And that stuff can really get tricky once you start digging into it... Certain things like harmonics can drastically affect how your audio sounds.   You might not want to "match" whatever the other bass-heavy songs LOOK like in the analyzer... Think of it as when you're at a concert, and you might FEEL the kick drum knocking your throat through your spine, but you might not be able to HEAR it very well necessarily... FEELING that thick kick sound, is what the low frequencies are going to do for you... But the harmonic frequencies above that kick's fundamental frequency, are the ones that will allow you to start HEARING the kick along with it... Sort of gets complicated if you've never dealt with that sort of thing before... But yeah, anytime you see a DRASTIC curve like that, then it's a safe bet that those low frequencies (that you can FEEL but can't HEAR) are going to trigger your limiter, before the audible high-mid frequencies do, and therefore, your song is going to SOUND quieter... (but it might make the subwoofer in your vehicle shake like crazy).  But realistically, as a starting point, everything below 80Hz you PROBABLY can't actually hear very well... so there is NO reason that the peak point of your song's frequency spectrum or whatever, should be down below 80Hz.  (Just my opinion)