The most probable reason is that you get a combined effect of different factors:
1. The vacuum makes the tissue flaps adapt better and more effectively. This means a quicker revascularisation (new vessel formation) and subsequently a better wound healing.
2. The material of the plastic catheter is considerably more tissue friendly than i.e. rubber and latex, which often are used in passive drains. The secretion caused by the presence of the catheter in the wound is therefore less when using active instead of passive drains.
3. With an active system, the diameter catheters can be kept considerably smaller than when using passive drains. A smaller surface of the foreign material in the tissue has a direct influence on the quantity of the secretion around the drainage and subsequently on the quantity of the exudate.
4. When using high-vacuum drains (>300 mmHg) it has been shown that there is often an excess bleeding. This is because the high negative pressure keeps the capillary bed open and prevents clot formation on the small vessels. With the low negative pressures in Bellovac/Exudrain/Abdovac the bleeding from the vessels stops earlier (faster healing) and there is thus reduced bleeding (and consequently a reduced need for blood transfusions)
One question that you can ask in connection with passive drains is: How correct is the volume estimation of the exudate? It is difficult to tell the amount of fluid in e.g. absorbent bandages.