Actually, NOT gates and Hex inverters are the same thing. Well almost!
A NOT gate is a buffer that inverts the output. A simple buffer does not invert and the NOT gate has a small circle on the output so imagine that as a little wheel that spins everything 180 degrees, it has one input and one output but putting just one such device in a chip would be hugly uneconomical so they include several.
So, a ‘Hex Inverter’ is just six NOT gates in a single package which of course makes better use of the space both in the chip, and also on the board.
Where a circuit has a couple of these NOT gates in series is often where a signal or pulse is required to have a small delay before it meets the next stage, which is only in the order of a few nanoseconds but enough maybe for another pulse of say synch or reset of a previous state to take effect. This is because everything you put in a signal path will give it some element of delay, however small. when you are dealing with high frequency or fast data rates, these delays can be useful or a complete nightmare so having a good dual beam oscilloscope when nothing seems to be working, often shows up where something turns up too early, or late.
In the particular circuit shown here, there needs to be some buffer but as non-inverting buffers tend to be electrically fragile, they have used two inverters in series (probably out of a 74HC04 or similar) for each line and the other two on the chip are simply not used.