The numbers represent the number of feet above or below the average mean LOWTIDE mark. For instance if you see a hightide listed as 3.5 feet as he stated, that would mean that at hightide there will be 3.5 feet more water above the average lowtide depth.
If you look at different locations on the North Shore & South shore you will see that the tides are more dramatic (6 to 8 feet) on the North shore and more mild swings on the South shore.
Also, if you look at the dates when there is either a full moon or a new moon you will see unusually low & high tides (negative numbers such as -1.2 would mean "less than the average mean lowtide").
These numbers are great if you like to fish in the flats. I have a few spots where there is only a foot or less of water during lowtide. I can use the charts to help calculate exactly how much time I have to fish on the flats before the water drops too low for my skiff. There are dozens of other uses for this information too.
No problem. The main reason why they print it that way is because all of our nautical charts show mean lowtide depth. You can then add or subtract the differential to see how deep it should be at any given location at the top or bottom of any tide.