The lunar nodes always move backward around the chart, but their real movements tend to wobble between retrograde and direct motion.
The mean calculation smooths out these minor wobbles by producing a mathematical average of the total movement.
The true calculation reflects the accurate location of the nodes at a given moment, rather than the mathematical average.
There can be discrepancies between true and mean calculations of up to 1°45’. For a smattering of people, that’s enough for the nodes to change sign and house depending on which calculation you use.
CHANI app uses the true calculation for the lunar nodes. If you’re used to seeing your chart from an astrologer or astrology software that uses the mean calculation, it could mean your nodes change sign.
While the true calculation represents the most mathematically accurate location, astrology is a field that tends to make space for less materialist modes of knowing. Just like choosing a house system, many astrologers will decide at the beginning of their practice whether to work with the true or mean nodes.
You can think of it like a chef working in metric or imperial. There’s no “perfect” conversion between a cup of flour and 136 grams unless you’re pouring that cup onto a measuring scale to get the accurate mass. Still, it’s likely to be pretty close.