Peak flow is nothing more than a crude means to establish peak flow. They are very much influenced by technique. It is therefore not unusual for people not used to doing it getting a less than representative value.
Lung capacity and cardiovasular performance are not very well measured by it as you so eloquently demonstrate.
Asthma is a condition which causes the small airways to be restricted (contracted muscle tissue around them does that). This mainly affects the expiration phase of the respiration process. The restriction causes a slow down of the air outwards and hence you will measure a lower value, in people who have active asthma.
FD