gabbott explained it well but I will add one more thing - as with all phones, the notifications themselves vary enormously in volume. Get the ZEDGE app, and try some new ones out. Some are literally twice as loud as any that come with the phone. Same for ring tones. The speaker is appreciably quieter than the 6P speakers though if by chance that's what you're used to.
Additionally, the volume rockers adjust whatever volume is most pertinent. If you're on a call, they adjust call volume. If you're connected to a bluetooth speaker, they adjust bluetooth volume. If the phone is doing nothing, it adjusts notification volume. Etc. etc.