Re: A few points and a question
@B, at the budget end of the scale laptops are currently cheaper than desktops, due to all the add ons.
If you want a basic machine for web browsing and word processing and not too bothered about much else, you can buy a reasonable laptop for around £300.
With this you get the monitor, keyboard, pointing device, Windows and it's portable.
Once you add a monitor, keyboard and mouse at this end of the market to a desktop it looks expensive.
Basic desktop machines with Windows (not trying to start a flame war here, but it's what most people are happy with!), are in the £300 region, to this you need to add a monitor etc.
Not looking so cheap now.
At the higher end of the market costs have been going up as well, the need for better airflow cases, larger power supplies and graphics cards going up in price have pushed costs up.
Not to say that an equivalent laptop won't cost more at this position laptops are much more expensive, but not as many people buy these machines. This will be reflected in the sales figures.
@ C Requirement dependent. Not everyone wants a bleeding edge gaming or processing rig.
@D The bulk of the sales are at the low end of the market and I doubt custom built machines will factor into the sales figures, this data will be from big manufacturers such as Dell, HP etc.