Stiegler/Magerovski were in the 40's in top PB's for the season based on last year's Skate America, which qualified them for consideration in any open spaces as one of the top 75. (I mistakenly and repeatedly posted that the last criteria was top 75 in ISU rankings, but, thankfully, chuckm corrected me.)
They had higher PB than Zadorozhniuk/Verbillo (PB NHK), selected for NHK, and Senft/Gisalson (PB Skate America), selected for Coc and NHK. (Japan chose three other US couples as well: Gregory/Petukhov, Davis/White, and Pratt/Giles.) They were twenty PB's higher than Rubleva/Shefer (PB Skate America), but USFS chose Gregory/Petuckhov, Matthews/Zavozin, and Navarro/Bommentre (whose personal best at Karl Schaefer was a handful of PB's above Stiegler/Magerovski). Huang/Zheng were in the 50's in PB based on Jr. Worlds. They received invitations to CoC and TEB. Carron/Jost were just one place above S/M in PB's, by .17 of a point, based on their performance at Karl Schaefer; they received two invitations, one to CoC and one to TEB.
In total, six US teams were given spots: B/A and M/G guaranteed two, Matthews/Zavozin, Davis/White, and Pratt/Giles guaranteed one, and Navarro/Bommentre guaranteed none, but on the potential list. Each got two.
Six Canadian teams were given spots: D/L and V/M guaranteed two, Lefebvre/Markov guaranteed one, and Senft/Gislason, Girard/Yeager, and Hann-McCurdy guaranteed none, but on the potential list. All but H/McG got two.
Seven Russian teams were given spots: Dom/Sha and K/N guaranteed two, Mikhailova/Sergeev, Platonova/Maksimishin, and Gorshkova/Tkachenko guaranteed one, and Rubleva/Shefer and Romanovskaya/Grachev guaranteed none, but on the potential list. All but Romanovskaya/Grachev got two.
Two Chinese teams were guaranteed none, but on the potential list -- Yu/Wang and Huang/Zheng -- and got two. Wang/Meng are a discretionary invite to CoC.
For France, the Del/Schoes were guaranteed two, P/B were guaranteed one, and Carron/Jost were on the potential list. All got two.
The only couple listed for Japan is Watanabe/Kido, who were guaranteed one and got two. The other spot for Japan, for NHK, is TBD.
It seems to me that Stiegler/Magerovski were not on the USFS radar, because there doesn't seem to be an unofficial limit of six per discipline. It may be strategy among the host nations based on the new selection criteria and seeding for Worlds to give skaters/teams two berths instead of spreading those out among two selections, because that will build up World standings points and give the skaters/teams an extra chance for a personal best.
From one point of view, the rich get richer. From another, the Federations are making strategic investments.