Electronic – Serial NOR Flash – Vague

The sector erase time matters to any driver because it is a bit of timing that is necessary to use the chip correctly. Flash chips have erase times, which is how long the special erase voltage must be applied to the cells to guarantee they are erased. If this minimum time isn't met then cells may not be fully erased, which is in effect a data error.

Some chips may also have a maximum erase time. Some chips require the external hardware to perform erase timing, others perform internal timing and set a status bit when erase is complete. In that case a driver would either have to wait the maximum possible erase time, or poll the bit to ensure erase has completed before proceeding to other operations.

As with any specification, violating it means that none of the other parameters can be relied on any longer. If this chip self-times erase and sets a bit when complete and the existing driver polls this bit to determine erase completion, then no modification to the driver should be required.

In any case, thinking you can "get by" violating a spec just because it's only off a little is very very bad practise and you should be ashamed of yourself for even considering it.

The best answer I've found to your question is covered at How Flash Memory Works where it says:

The electrons in the cells of a flash-memory chip can be returned to normal ("1") by the application of an electric field, a higher-voltage charge. Flash memory uses in-circuit wiring to apply the electric field either to the entire chip or to predetermined sections known as blocks. This erases the targeted area of the chip, which can then be rewritten. Flash memory works much faster than traditional EEPROMs because instead of erasing one byte at a time, it erases a block or the entire chip, and then rewrites it.

I don't understand why the "in-circuit wiring" allow for bit level programming (switching from 1 to 0) but it might be related to the different way the transitions 1 to 0 is performed (programming via hot injection) compared to 0 to 1 transition (erasing via Fowler-Nordheim tunnelling).