Don't worry. Your $$$ isn't totally lost. You were effective at reducing the reverberations within the front area. That's the majority of engine noise anyway.
Your meter measures SPL or Sound Pressure Levels. If you've ever been to a sound off, this is the measurement to see which car puts out the most audio. They do this with woofers and subwoofers that producer extremely low BASS. You've heard it at stop lights numerous times. So, when you are measuring the SPL of your bus engine, you are measuring all frequencies together, but the bass or low tones is typically what increases or decreases the SPL.
Your ears haven't deceived you. If it sounds better then before, it is.
Enjoy your new sound.
Learning by experience isn't always the most fun, but it's effective.