It is erratic indeed. The post office gets 1000s of international parcels a day and they can realistically only perform a spot check of those. If the parcel has clear and honest valuation paperwork attached, you're probably going to get stung though.
In my experience, the couriers who offer an end to end service (say UPS between the States and the UK) are more likely to charge you because they have greater visibility of the parcel contents/value.
Having said that, there is a potentially major benefit in using an end to end service and that is traceability. I once imported a guitar from the US using national postal services. The guitar got lost at port of entry for nearly 3 months - US Post would not take responsibility as they had a record of the parcel leaving the States and HMRC/UK Post wouldn't take responsibility as they had no record of the parcel entering the UK. Wonderful... I've stuck with end to end services for anything valuable ever since then.