... line sponge is very real thing and I remember another little experiment I did which showed up the very thing you mention, heavier steering.
I went to a local yacht shop and bought the best line they had to replace the single line that goes from the depower strap to the front Y point. It turned out to be really spongy and each dive of the kite, the kite would distort and the bar got heavy. If I dived the kite slower and let it creep up and down it wasn't as bad but load it up, it got worse. What I concluded was the kite angle of attack changed and the kites back lines were too short during the flexing, choking the kite. If you choke the kite, steering gets worse and you notice this as heavier bar pressure. Without the line flex the AOA stayed the same and therefore wouldn't choke and steering is normal.
There must be some stuff or treatment on lines that must wear off after time. I noticed that after a few months the lines would get little fibers hanging out, sort of looks like it gets furry. So I figure that if the outside parts of the line wear, maybe the inside fibres are wearing down too making the line thinner by enough to cause the flex we feel. Over 20 to 30m of kite lines, this is enough for us to feel it as spongy lines.