> For some serendipitous reason, even though the disks are different
> models they have exactly the same geometries.

No serendipity.  It's a general industry trend.

The geometry you see is fake.  Any modern drive is zoned so a simple 
geometry can't describe it anyway.  The drive firmware translates 
the internal geometry to/from a simple fake external geometry that 
it tells you about. 

Rounding the total size down a bit to match a "standard" size means 
that you can do things like replace drives in a raid set or do a 
disk-to-disk copy and have everything work right without having to 
use a the same manfacturer/model target disk as the original one. 

Think of the others as being clones of the "standard" if you prefer.