Some of the drug will always be absorbed orally and travel to the liver through the the portal vein.

The liver will eventually remove ketamine from the bloodstream regardless of which route is used.

That's my understanding at least.

That’s interesting. What’s the mechanism for that? Uninformed intuition would lead one to think there would not be a difference.

It's a function of anatomy. The sublingual veins drain into the jugular and then vena cava. For drugs this avoids the first-pass effect, which is exploited when giving nitroglycerin for angina pectoris. Other ways to achieve this are suppositories (rectal veins) and intravenous (i.v.) or intramuscular (i.m.) injection.

https://en.wikipedia.org/wiki/Lingual_veins

https://en.wikipedia.org/wiki/First_pass_effect