I tend to use such phrases as a shorthand for "I can say that X is true based on my own experience, but have not explicitly researched this topic and so am open to disagreement from someone more knowledgeable".
On the other hand, expecting such nuance to come through in the phrase "I'm almost positive" or "nearly certain" (my preferred variants) is likely a bit unreasonable.
On the other hand, expecting such nuance to come through in the phrase "I'm almost positive" or "nearly certain" (my preferred variants) is likely a bit unreasonable.