For EU868, the SF would indeed be the same, and I’d expect that for other regions too. But peeking into the specifications for AU915, the SF is indeed different:
…along with:
I’ve not tried to understand those… TTN might be wrong what does it say for a Join Accept that is actually received well by your node? but as it seems to work, I guess it’s okay as the node agrees with this… (Also, it might use a higher transmission power for the downlink, just like TTN does for RX2 in EU868, but that’s just a wild guess.)