I just googled passata and it said that they was no fibre in it !!!!
So thats that theory out of the window. Unless you know different.
Well, yeah I do know different, having done a degree in medicine
But of course google is the God of all knowledge innit.
All vegetables are fibrous - that's what they're made of, in the main.
Basically there are three main parts to your food - carbohydrate, fat and protein. Vegetables and fruit are mainly carbohydrate (with varying degrees of protein too but minimal fat).
Carbohydrate can be complex or simple. Simple carbohydrates are basically sugars - your body digests them easily with very little effort and turns them into fat or uses them as energy. Complex carbohydrates are a lot harder for your body to digest and could use up more energy to digest than your body could actually get out of them. Fibre is made of cellulose which is a complex carbohydrate, a lot of which your body can never digest so it comes out the other end.
A tomato is made of lots of carbohydrates - both sugars and cellulose (fibre). A LOT of fibre. Tinned tomatoes are mushed up tomatoes so a little of the breakdown work is already done and there is less fibre than in a standard tomato. Passata is even more mushed up with even more of the work done so less fibre again. Tomato juice has had so much bloody work done that hardly any fibre remains as it has been filtered out. It's almost all sugars.
It's no different to fruit vs fruit juice - you dont have to syn an apple because most of it needs work to digest and doesn't have all that many calories - its all water and fibre. But apple juice has had almost all the fibre taken out already so there's very little work left for your body to do and it just sits there and takes in all the sugars (there's *nothing left* to 'come out the other end') - so you have to count both fruit and tomato juice as syns as the healthiest stuff (the complex carbohydrates, mainly fibre) has already been filtered out/removed.
The above is a gross oversimplification but gives you the general idea I hope. It's better than the top hit of a quick google anyhow.