I think the problem with those measurements were the fans I used. The models were similar (looked identical but had no tecnical label on them) and over 10 years old, so I guess the air flows were unbalanced.
There were no ducts except for a piece of pipe (120mm diameter, 1mt long) that i had to use for the hair dryer to prevent the polystyrene casing from melting down.
As for the distanced PP core's performance I think the issue is the lack of small channels in the distanced flow which allows more air to pass through exchanging smaller amounts of temperature.
Sorry but I no longer have this HX as I used the material to built up the bigger ones I currently use.
@ ham789: thanks for your suggestions. I'm still waiting for that anemometer to arrive from the bay... I'll try to measure air flows as soon as possible.