As other people have said, healing parses are a bit different. If you have a terrible group standing in everything bad, then your healing parses go up because you are forced to over compensate for their fail damage. Theoretically, if everyone in your group does things correctly, tanks mitigate correctly, and people perform properly, your healing will be less needed, and thus your parses will seem lower.
As a healer, I wouldn’t put too much stock in parsing. If you wanna use the tool to compare yourself to other healers, and utilize it as an informative tool to see where people might be doing things differently during certain spots of unavoidable dmg (think Queen’s Court when all the AoE is going out that people can’t avoid), then that’s one thing. But don’t worry about it too much.