Quirks of the FPTP system; it giveth and it taketh away.
Last year, Labour won lots of rural and suburban constituencies where they've never had MPs elected and have little organisation, because voters were sick of the Tory government, and because the rightish vote was split.
Labour also won back the red wall seats they lost in 2019, but on low turnouts and with no enthusiasm.
All those seats would be vulnerable with only a small reduction in Labour support, never mind the tanking we've seen since last summer. There's a by-election coming up in Runcorn, by some metrics one of Labour's safest seats, and Starmer has conspicuously not gone there.
The megapolls and MRP projections are showing that, outside of inner London and Liverpool, there really aren't very many safe seats left for any party. There look like being lots of very tight three-way races.
Due to the quirks of the system, and Reform having support that's quite evenly spread rather than Lib Dem style regional strongholds, you could have Reform at 25% being the biggest party in the popular vote but in third place in terms of seats. Or you could see Reform push up to about 28% and start winning everywhere.
And if there's anti-Labour tactical voting or they leak support on multiple fronts - to Reform, the Greens, the SNP, the Hamas independents - then Starmer really is fucked.