Assuming they use the standard set of elevation data produced by NASA (I think), elevation data is only available to a resolution of 90 metres. This can often explain missing hills / dips as the sample points do not actually align with the route or points on the hill.
It’s also possible that when calculating the gain, they take a number of samples at points along the route that is slightly different to what they use to draw the elevation graph with. With would lead to the discrepancy between their total and your calculation.
Not wanting to interrupt your questions to Brian, but I struggle to understand how your route is any better. The first 9 miles are all along dual carriageways / main roads, which are detached from the communities of Preston. You also have a similar 5mile stretch around Eastway / Tom Benson Way, and although this is closer to Preston, is still detached from most of the population.
The route also crosses / runs along every major route into and out of Preston, so we can either run with the traffic, or prevent anyone entering / leaving Preston on the day.
At least heading out into the countryside may provide something more interesting to look at than the long road ahead. I don’t expect people to be lining the streets where ever the route goes, but at least we might get a bit of encouragement on the way through the villages.