I think some of the ratings are incorrect - 100W GLS puts out 1340 lm standard, possibly somewhat more for a Krypton filled lamp (as might be for a mushroom lamp), or somewhat less for a coated lamp. It probably is brighter than 980 lm. Or it is a long life lamp with 2000 hours or more

I never noticed that, I’ve just got some Philips 100 watt mushroom lamps so I should really try them side by side to see how the brightness compares

Yes, if correct the lumen output is terrible for a regular 100W lamp. I have never heard of this brand before and suspect that despite the shape is not Krypton filled. The coating will knock some lumens off but it should not be anywhere remotely near the sub 1000 lumen figure.

Also the size of these lamps, they are considerably smaller than the usual 100 watt mushroom lamps