… and what is the significance of the grouping?
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That table seems to be mangled somehow...
From the fields version and IHL you can tell that those numbers indicate the bits for those fields.
For type of service that continues but the first digit of the two-digit numbers is somehow placed in the upper half of the column header - so the indicated bits or 8 through 11 and 12 through 15. That scheme continues with 16-19, 20-23, 24-27, 28-31.