Bug? It looks correct to me:
4 x 10^-5 = 0.00004
and 0.00004 = 4 x 10^-5 or 4e-05
Ooops (ikegami & toolic bring me to my senses):

DB<3> print Number::Format::format_number(-0.0004,5)
-0.0004

Peripheral note: strikeout makes "=" look like -.