Per-ip shaper and shared shaper in the same policy

Hi to all,

consider please my scenario. I have 2 interfaces: ether1 connected to a private lan (10.0.0.0/24) and wan1 interface connected to the internet with a public ip address. The wan link is at 100 Mbps in up and down but I would to assign to each user the same bandwith and I would limit the total traffic at 20Mbps. Then I created a shared traffic shaper with max-bandwith of 20000 and a per ip-traffic with max bandwith at 0. Then I assignet it at the same policy. 

I have done this tests:

1) I tried a speedtest from a pc in 10.0.0.0/24 and it is ok: the maximum bandwith in up and down is about 20Mbps

2) I tried a speedtest with 2 pc in 10.0.0.0/24 but in the same time: I expected about 10 Mbps for each pc but it isn't. I tried to launch speedtest in the same time but obviously it is difficult but there is about 1 minute in which both pc are downloading in the same time. The result is: the first pc started download report about 15 Mbps and the second about 5Mbps. 

Why? 

L.