Brown Ale is usually split into two major substyles: Southern and Northern. These are usually typified by respectively Mann's Brown Ale and Newcastle Brown Ale. Or, put simply, Southern sweet and weak, Northern dry and strong.
But when and why were Brown Ales classified this way? The first question is pretty easy to answer: by Michael Jackson in his World Guide to Beer. As for the why, well probably because there were by then a dwindling number of examples and the two best-known beers called Brown Ale were pretty different from each other.
Does this differentiation have an historic basis. Were brewers making two distinct variations of Brown Ale?
Maybe the Whitbread gravity book can give us an insight into what Brown Ales were around just after WW II. There are a lot of Brown Ale entries. They have a good geographic spread, too, ranging from Scotland to the south coast. Here, take a look:
You'll notice that they are a fairly heterogenous bunch. Their gravity ranges from 1025.6 to 1070.6. The degree of attenuation is just a diverse: low 57%, high 85%. As would be expected, two of the stronger, drier examples are from the Northeast: Newcastle Brown Ale and Vaux Double Maxim. But in amongst them are a couple from Portsmouth. It's hard to get more southern than the south coast. Hang on a minute, amongst the weak, sweet ones are northern beers: Sam Smiths and Tennants of Sheffield.
I'm not going to start preaching. Look at the numbers and draw your own conclusions. Yes, there are beers like Newcastle Brown (the beer itself is there) and Mann's. But there's also just about everything inbetween (and even above and below) those two.