Those a grass seed stems/stalks. There is nothing wrong with your mower. I have never seen any mower, blade or combination that will mulch them. They will be gone by mid July.

Can you explain why I have this and why others don't? Are there anything I can do to reduce the number as it seem to be a lot of dead grass over a couple of months? Why will it be gone in July?

Thanks

"Can you explain why I have this and why others don't?"

Not really. Do they bag their cuttings? Are their lawns as thick as yours or are their lawns thin and the stems fall through the canopy? I doubt that it is because of their mower.

"Are there anything I can do to reduce the number as it seem to be a lot of dead grass over a couple of months?"

Some have tried applying growth inhibitors with less than stellar results. The real problem is physics. Seed stems pull out easily when the blade hits them and their tube like shape is not conducive to being suspended by the mowers suction for subsequent mulching. Pull on a seed head and see for yourself how easy they pull out. Compare the resistance to that of a blade of grass that is attached to the turf.

"Why will it be gone in July?"

The grass's seeding cycle should be over by the then.

You could bag the grass until the stalks go away. Mowing more often would also reduce the problem. A really thick lawn requires very frequent mowing when mulching, at least with bluegrass