Yes, the smaller number is based on GURPS and the larger comes from drawing the data through Striker's numbers for planetary GDPs. There might be some small variation because I noticed some of the world data changed from the original sources when T5 came out, but I don't recall any change in the big worlds, and they dominated the numbers.
I don't have Pocket Empires, I do have Keepers, but I was focused on the FFW era so never tried numbers from Keepers. I wonder how much of the difference relates to the different eras. I believe one of the big worlds got blasted pretty badly between the two eras. On the other hand, with GURPS and Striker producing a factor of 3 difference, it's as likely to be due just to the different methods of computing a world's economy.
I have long entertained the idea of rolling the clock back a hundred forty years and setting play in the pre-3rd Frontier War era. Slash most or all world populations by an order of magnitude (maybe populate some of the currently empty worlds with small colonies that would eventually die out), cut the Imperial highest tech level to ... 13, isn't it? Reduce the system tech levels generally, and so forth. I imagine there are resources on the TFW period, but I don't have them. It would seriously reduce the wealth available to the Marches and put them in the era when privateering and border skirmishing in the lead-up to the war would have been more prevalent - and, just as important, when the reduced populations meant a greater reliance on free traders and small transports.