For anyone interested, it this is the response I received from Pinnacle:
Thank you for contacting Pinnacle Customer Support.
For certain Handicaps, Pinnacle will move the base Handicap for a Teaser to a different Handicap. This is because in certain situations, we must limit our exposure and thus cannot let unlimited Teasers to occur at certain prices.
In all situations, we reserve the right to move our prices to limit our risk.
As per our rules, we reserve the right to determine the base spread to be used for any Teaser bet.