Are different mainstream Lagers really all that different? Is there any differences in the mainstream company lagers?

I often see people go into bars and just order a pint of lager, even though there are many different lagers available at the bar. This leads me to believe that most of the mainstream lagers don't actually taste that different from each other.

As I don't drink lagers — I tend to stick to cask/real ales — lagers all taste similar to me anyways. 

So is there really any difference with these companies' lagers? 

P.S.
By mainstream mean Fosters, Carling, Carlsberg, etc...
Most generic lagers have subtle differences, but in general as you say they are usually very similar.

To be frank, though, this is something I find with just about any style of beer. For instance, if I'm buying an imperial stout from any given brewery I can reasonably expect what the beer is going to taste like for the most part. The only thing that really brings it down below my standard is if it's just not a quality beer.

In the same way generic lagers are an equivalent style and have a similar purpose, to be non-offensive and give people the 'beer' taste that they expect. So if you're drinking a Canadian, to a Bud, to a Sapporo you can expect them to be pretty similar.

The one additional thing I'll say, though, is that when beer falls into the realm of 'craft' you usually see a higher range of flavour within a style, than with generics, though. This is because the very purpose of a generic lager is not to stand out as to gain mass appeal.