I think Victoria is concious of the fact that her smile is not very attractive when photographed - plus the photographers tend to catch her in repose, when she may or not be being serious, just appearing to be. This helps them builds on the imagined personna that she is miserable, she and David are unhappy, she is ill / anorexic / bullimic / weight obsessed and so on.
The fact is, the Beckhams are still together, and still appear to be happy - but that's not what the press want. Tabloid and feature magazines thrive on conflict - the underlying notion that being rich, successful and attractive still makes you miserable - which is what their readers want to read, it makes them feel better.