The only reasons I can think of why it might smell more for some and not at all/or very little for others doesn't really have to do with the fleece but has to do with the pigs themselves. Pigs can pass gas if fed gassy foods and they can have more smelly urine depending on what they are fed. Boars mark their territory and have their own smelly problems, sows can spray their cagemates with urine. The only problem I can think of with fleece itself is using more than one layer. Pee will sometimes get "trapped" in the layers and cause smell. I know from experience.
Of course for the people who may not spot clean daily/every other day, it's probably going to smell. I have used fleece for 8+ months now and there was only 1 time that my cages even slightly smelled and that was when I was gone for 2 days and didn't spot clean. I have 5 sows and 2 boars. My 5 sows can produce a huge amount of poo in just one day so I have to spot clean at least once a day.
I use about a 1/4" thick layer of newspapers under the fleece, then I use a sheet or mattress pad on top, then one layer of fleece. I generally clean every 5 days but I can leave cage cleaning go for up to week.