Why does smoking CBD buds have a different effect compared to CBD oil?

There are two part-questions to this question. First: Why do raw buds have a different effect to concentrates? And second: Why does smoking have a different effect to ingestion?

In order to extract the CBD (cannabidiol) from the plant, either by creating CBD oil or by smoking it, you need to decarboxylate the CBDa (cannabidiolic acid) by heating it. When you smoke the raw bud, the heat of the flame and embers is decarboxylating the CBDa as you're inhaling it, but it's not a perfect process. Not all of it is going to be decarboxylated, and it won't be the same every time due to the variability of how plants will grow. Similar to how all oranges will grow slightly differently, so too will all cannabis buds.

Concentrated oils are far more accurate and controlled. In one jar of oil, the same dose should lead to the same effect each time (assuming no tolerance is built up), but the same cannot be said for every puff of a bud.

Addressing the difference between smoking and ingestion, smoking buds utilises your body's respiratory system in order to extract the CBD directly to your bloodstream. It means a faster path to your brain, but because of that it also results in a shorter duration of effect and a more rapid drop after the effects have worn off. Ingestion of the oil utilises your body's metabolic system, which means a slower path to the brain but a much longer duration of effect with a more gradual drop after the effects begin to wear off.