I've found when hot-packing jam into jars that have just come out of the oven at 160°C, the jam that is in contact with the glass keeps cooking and thickens. You can tell because it sizzles as it goes in, despite being over 100°C when poured (I use a jam thermometer so can be fairly sure). Letting it cool then packing for rapid consumption always leads to thick jam for me, as it carries on cooking while the rest is packed. But this tends to be the last bits that aren't worth packing properly, so gives me an early taste test portion, if a poor indication of texture.
Letting the jars cool a little would reduce this, in fact if I take several jars out of the oven in one go then fill them the sizzling is reduced by the last jar.
After hot packing into sterilised jars you don't (according to the UK food standards agency) need to carry out a further step, so I don't. Even pressure canning would have a limited effect on the water content of the jam overall, but bay cause some further cooking against the glass. Simply boiling the jars (as advised in old books for preserves with lower sugar content) should have no effect, as the jam was already hotter than that.