As others have replied, the "e" is a metrological passport which permits it to be specified as average weight if exporting so unnecessary in the UK but all UK weights (unless the manufacturer is daft) are average nowadays.
For a multipack of crisps it matters how the manufacturer has declared the weights. Two options are available to them one gives more tolerance than the other. This is a typical multipack fudge by the way. So a multipack has more leeway than a sharing bag typically.
But this seems like it was a sharing bag rather than a multipack with a declared weight (the weight with the e after it) of 154g.
The pack is between 100 and 200g there is potential for up to 4.5% of error for T1 and double that error for T2 (I'll explain in a mo).
Basically that means the net weight of your bag should have been 140.1g minimum (that's the T2 weight) but only 1 in 40 bags should have been between that weight and 147g. The overall weight of the batch should also have been above 154g on average.
So to summarise, if your scales are accurate, that bag was illegal. If you are using digital scales and have a known weight around that's worth checking them but also I'd drop a note to the manufacturer. Even if you don't have evidence as that shouldn't be happening.
Most manufacturing plants will have checkweighers in their plant. These aren't perfect but they're ok enough and certainly shouldn't be letting over a pack of 120g. They are conveyors with a weigh cell underneath.
What can happen in poorly set up manufacturers are two things. I suspect the latter as it's more likely to give a big error.
- They don't set up the check weigher properly and account for variability in the weighing. (Common but only usually causes minor errors.)
- They don't set up the acceleration belt properly before the weigh cell which basically means that two packs can go across at once from time to time.
Also though those kinds of bags are filled using multi head weighers which in themselves are normally pretty accurate. Something has gone pretty badly wrong at the manufacturer for this to happen if your scales are bob on.
Contact the company who sold it. Say you've not contacted trading standards so far as you'd prefer them to look into it themselves. That should be enough for them to give you a voucher and look into it properly.