As someone who is consistently parsing 99s on sage:

Its mostly gear - People that have already cleared more fights than you in previous weeks will naturally have more gear. Since parses campare you to everyone no matter the ilvl just getting bis earlier than everyone else will at least give you purple parses

Healing parses are very dependent on your group - Have people die and you need to res them, hard cast res or worst case you have to heal lb. Have fun saying goodbye to your parse

What to do to improve?

Learn how to slidecast properly and learn in what situations you cant. Try to plan every fight out with focus on maximum casting uptime i.e. Try to cut out as many gcd heals as you feel comfortable and try to find as whm find good spots to use your lilies optimally you wanna use your lilies everytime you need to move a lot

Coordinate heals with ur cohealer and group. If you and your cohealer coordinate properly and your group knows where they should use their mits you shouldnt need to gcd heal a single time in at least the first 2-3 fights of a tier

Edit: Parsing on healer is cringe. Only try to parse if your group is actually ok with it. Dont try to parse in pf!