The problem is 'balance'.

When designing an accordion to manufacture, decisions have to be made about the 'balancing' of the instrument. It is like a pipe organ in that respect. If the player 'pulls out all the stops' the instrument needs to be balanced so that neither melody nor bass is drowned by the other. With a simple instrument with no stops or registers, that is relatively easy. The more reed banks on either side that can be brought in or left out at the players whim, the more difficult the design of the balance needs becomes.

It also depends on whether the 'piano accordion' is a 'Stradella Bass' or a 'Free Bass' as the two types are played quite differently. If is is a combination of the two, that makes even more design demands.

It also depends on whether some reed sets are chambered (Cassotto) or not as this also affects the balance when the chamber is opened or closed.

Now the more complex instrument is designed to NOT always be 'in balance' between the two sides with all combinations. This is for several reasons: the player may play either side individually, but more usually just the keyboard side; the player may wish to have one side dominate over the other - sometimes it is desirable to have the 'bass side' sound louder than the 'melody side', and vice versa.

Playing technique also affects 'balance'. Just droning away with both sides is for beginners. One can 'vamp' or play staccato either side while playing legato on the other side. This 'softens' the vamped side, as the reeds do not speak long enough to build up the volume to their maximum output.

I could rant on like this for ages... but look at the thread Technique: Piano Accordion for The Recycled Muso, where I already have... :-)

Robin