Ah, it seems to be the distance. My track 5k is also quicker than road 5k but the other way around for 10k. Can’t think why it would be like that.
Based on my best 10k time last year my equivalent road half marathon time is 1:30! I reckon that’s about 10 minutes quicker than I could do it (although I haven’t actually tried) so I think these calculations shouldn’t be taken too seriously.