Scientific Notation question

[Martin Green]

Unicode LDMR reference says this about Scientific Notation patterns:

  * When using scientific notation, the formatter controls the digit
    counts using significant digits logic. The maximum number of
    significant digits limits the total number of integer and fraction
    digits that will be shown in the mantissa; it does not affect
    parsing. For example, 12345 formatted with "##0.##E0" is "12.3E3".
    See the section on significant digits for more details.

I don't understand how they arrive at this. From the result of "12.3E3" 
I assume they have determined the maximum significant digits in the 
pattern to be three, but I just don't see that. My expectation would be 
that "##0.##E0" would format 12345 as "123E2". The only thing I can 
think of is that perhaps scientific notation MUST have a fractional 
part, which would mean that the result  for "E2" would be "123.0E2" 
which is FOUR significant digits. Since I don't see what made them 
decide there can only be three significant digits I don't get why 
"123.0E2" would be an invalid result.

What am I missing?
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