Why songs on the playlist sound so different, so unequal?
Why is that so? Here's why:
The tools: The tools for audio mastering are coming from various periods. Older songs mastered with older tools usually sound more "closed"/muffled while the new ones sound more direct and pushy (sometimes even excessive). Not to mention the differences between high fidelity digital tools in comparison with lower but warmer analog ones.
The people's perception for sound: There are as well thousands of different people doing mastering with various tastes for hearing perception. Thousand people, thousand tastes, thousand various sound blends.
The mediums: Two main medias until today (vinyl and CD/digital file) can sound different. Besides the EQ can sound different (unequal amount of bass, vocals, mid-highs and highs), when vinyl is worn out, the high frequency begins to disappear slowly (example). The vinyl usually has more dynamic sound and in rare circumstances reach higher frequency in popular music, but on the other hand, the amount of high frequency is limited on vinyl, while on CD/digital file is nearly unlimited. Also, did you know a bass up until 300 Hz has to be in mono on vinyl?