parcel - was i just unlucky?

I'm just wondering if anyone had the same problem.
I bought some aromatherapy oil as a Christmas present for someone in the UK. It was 250ml.
The Central post office rejected my parcel saying anything over 100 ml of liquid cannot be accepted. They asked me first if it was parfum, I said no. Then they told about me that the liquid rule also applies to parcels. I know for a fact that my helper and her friends regularly send shampoos, face gel etc in parcels (without declaring). They never get asked (so I was told).
I called DHL to see if I can take another route and they said if over 100 ml they want an independent lab test to verify the contents of the liquid!
Ultimately I found a solution around this problem but when I went to another post office to send this parcel, they didn't even asked if the parcel contained liquid (let alone its volume!). I wondered if technically I could have just send it off without declaring 250 ml liquid inside. Of course, one could argue if passed through an x-ray it could reveal itself but then how could so many other parcels with liquid gotten through?
Has anyone had this problem?