Cable Capacities - Self Build Wiki

current capacity depends on method of installation. (Earth impedance may come into it too.)

NT

I reckon it's a mistake. Look at the other sizes of wire:

The current flowing in line and neutral should be the same. Especially if you have an RCD. ;-)

I'd say it's a typo. Unless you know what the conditions of use are they are the same according to my tables. Or even the TLC site. ;-)

Of course singles are frequently bunched within conduit or trunking.

Consider the overall circumference of the cable. The plastic is a very poor (thermal) insulator.

This means that essentially, the copper size is irrelevant to its thermal properties (of course, it's important with regards to how much heat is generated in the first placE), the only thing of importance being the overall circumference of the cable, and the environment it's in.

If a 1mm^2 copper conductor is generating 1w/m^2 of heat along its length, it's going to be cooler if the heat emitting surface is 30mm(/2) (guess) of insulation, rather than 11mm.

It may be down to the fact the surface area of the single is less than 1/2 that of the flat twin.

Incorrect, out-of-date or inadequately specified cable ratings abound on the Web, so be careful. In fact it is rare, IME, to see correct figures. BS 7671:2001 + Amd 2:2004 and the On-Site Guide are your friends.

In this case it looks as if comparison is not being made between similar installation methods.

The up-to-date clipped direct rating for 1 mm^2 T&E (6242YH) in a single phase circuit is 16 amps, or 11.5 amps if enclosed in conduit in an insulated wall (with the conduit against studwork or masonry. [BS 7671 Table 4D5A]

For singles we use Table 4D1A which gives 15.5 amps clipped direct (which would apply for 6181Y sheathed singles) or 11 amps in conduit in an insulated wall as above (& would normally be 6491X insulated singles as in an all-conduit/trunking installation).

When properly compared the ratings are very similar.

No, the ratings are always for a _circuit_, either single phase (2 wires or cables) or 3-phase (3 or 4 wires or cables). (For a ring circuit interpret this as the rating per 'leg' of the circuit.)

HTH