Because if you fill those extra 2 hours Bob's managed to accrue with more work Bob will be sure to inject a few more HN/Reddit/etc visits into his 8 hours next

Because if you fill those extra 2 hours Bob's managed to accrue with more work Bob will be sure to inject a few more HN/Reddit/etc visits into his 8 hours next time. There's rarely any motivation, gain or benefit in taking on extra work in your standard 9-5 job