Strange questions, "why"?
Different instruments are naturally louder or softer than other instruments, different phrases can be stored in Arpeggios and will naturally play at different levels... remember you have control over the volume of each Part. Velocity is also an influence on how loud an ARP phrase plays.

You only trigger the ARP phrases, it is not "you" playing, it's MIDI data... each ARP phrases can either playback at the velocity as stored in the original phrases or you can set the ARP to playback at velocity of your controlling trigger notes. That is your choice.

Each Part has a parameter set that determines exactly how the assigned ARP phrase will behave.

Whether an ARP phrase plays continuously is also your choice, you set whether HOLD is On or Off, you also can control whether you touching the keys cause the phrase to "gate" (play as long as keys are held), or "toggle" (play until you touch the keys again). Your choice.

Why? Because that is the most musically useful way to apply the ARP phrases within your music.