Because people raising one breed called it one thing, and people raising another breed called it something different. They did develop in different places at different times, before everyone was communicating over the internet.
Speckled--it looks speckled
Jubilee--I've heard the same as Jenwisp:
Mille Fleur--French for "Thousand Flowers" (yes, "mille" is thousand in both Latin and French, even though it sounds like our word million. A million is a thousand thousands).
All three of those have the mottling gene making the white and black dots.
Other names for chickens with similar mottled patterns:
Swedish Flower Hen
Tolbunt
Spangled (Old English Game Bantam, Cornish, Russian Orloff)
Overlapping names:
"Jubilee" Cornish are presumably named for Queen Victoria's Jubilee too, but are white double-lacing on red, with no mottling.
Spangled in Hamburgs and Spitzhaubens and Brabanters has a different genetic cause.
When a chicken would be brown (any shade) plus black, then mottling makes a white feather tip, with a black marking behind that, and then the rest of the feather is colored normally (according to the other genetics the chicken has.)
When a chicken would be all black, mottling makes a white tip and then the rest of the feather is just black. (Examples: Ancona, Houdan, any chicken variety actually called "mottled.")
Any gene that affects how the brown or black looks can affect those colors:
--Mille Fleur d'Uccle has a gold-toned brown
--Speckled Sussex has Mahogany and maybe other modifiers making the brown darker
--Porcelain d'Uccle has lavender lightening both the black and the gold
--Pearl Old English Game is black with mottling, then lavender lightening the black
--Golden Neck Old English Game had Dominant White changing all black to white, so it's got white dots on a gold bird
--Silver Mille Fleur has the gold turned to white, so the birds is white with little black v's near the tips of the feathers. (Photo on this page:
http://www.belgianduccle.org/silver-genetics )
--Blue would turn the black to blue, although I cannot think of a variety that has this.
Given how many different appearances there can be, it took a while before people figured out that the mottling gene was involved in all of them. (I think there are technically multiple alleles of the mottling gene, but since they are all at the same locus, and are all recessive to not-mottled, and seem to give similar results, I'm being a little bit lazy by referring to "the mottling gene.")
