Yes, this is a perennial issue and frustration. As discussed ad naseum in some threads last year. When objectively measured, say from the middle of the hip belt to the place where the strap attaches to the pack, two packs from different companies (and sometime from the same company!) labled as 19" can have completely different measurement, neither of which is "19". LOL
On top of that company wants to have a few models as possible to cover the consumer spectrum for financial reasons, so they will often fudge the "recommended" range of torso size up, especially on either end of the size spectrum, so they don't have to make as many different sizes. This lead to ridiculous statement on the label like "fits torso length 14" to 17.5 inches". You can be sure that they will rarely recommend any potential customer to NOT buy a pack in one of the available size, so your torso size is going to be "in-range" somewhere regardless. ;-/
So by the time you measure your torso in the prescribed way, that number may not tell you too much, especially when you factor in you personal foibles, body shape and preferences.
The best advice is to use the torso length just to get you in the ballpark for trying things on, and completely ignore it after then. You should look mostly at the wrap around distance (how many inches below, or sometimes even above, your shoulder tops do the shoulder straps attach to the pack. You should be looking at this even if you don't swallow my advice that you should be trying to minimize that distance.
There is one way that worked for me perfectly last year to get a more or less reliable fit if you are ordering online, provided you can get the data from the pack makers. If you already have a pack that you have long experience with, and that you feel fits exactly the way you want (I guess in this senario that pack should be falling apart since it is SO great, LOL) then measure THAT pack from the place where the straps connect to the pack down to the middle of the hip belt. You could alternatively try to figure this out from packs you try on in the store, but the ideal way is if you have used one for real.
Either way, when you measure that pack THAT is your personalized "torso length". If you have a pack, but you think it is 1/2 inch short then add 1/2inh to that measurement. This measurement will often still mean almost nothing in relation to official torso lengths suggested by the company. However, it DOES mean that any pack (of basically the same kind) that has the same measurement from the place where the shoulder straps attach down to the middle of the hip belt WILL be a great fit for you. This at least is an actual quantitative basis for a fit. One you think yo know that number for you, the forget EVERYTHING else.
All you need to do is then get that exact measurement somehow from the pack maker, or over the phone from a dealer and you are set. You could also measure the pack in a store this way, but there you have recourse to trying them on. Whether you ask someone over the phone, of measure yourself, make sure you get the actual measurement, NOT the suggested torso range, or anything else with a fudge factor. If the two number are within an inch of each other the fit should be ok. Best if they match exactly. remember using this method completely subsumes any variability as to how much over wrap you like and so on. The ultimate arbiter for you is that original pack you know is great.
Hope this helps somewhat. It worked like a charm for me last year. Both packs I bought were from cottage industries an it was easy to get those exact number over the phone foe each size they offered. for more commercial pack you can measure for your self in the store.