The explanation of the 'build up' is to explain why the payout starts off lower.
The details of that are that each share has a chance of being paid multiple times in the past 500% and if you own those shares that's all that matters.
If the share exists for the full 500% then of course it's chances of getting multiple payments are higher.
![Tongue](https://bitcointalk.org/Smileys/default/tongue.gif)
Of course all shares exist for 500%
The correct statement is that as you have more shares in the last 500% then you will get more per payout.
If you have mined for 1/4 of that 500% then your peak shares in the 500% is of course 1/4 of the maximum.
Once you have mined the full 500% then your share of the total payout will have reached it's maximum for your hash rate.
Each share itself still gets paid what it can, but your total shares (in the 500%) determines your % of the reward.
If you own shares under different accounts, it's still the same sum total payout to your address or addresses.