Bought the Equate brand at Walmart over 4 years ago and changed the batteries once. Always bring it with me to Dr's appointments and it is right on with theirs. Cold fingers, hand creams, movement can all give incorrect readings as Michael said.

I havegood one by OxyWatch from Walgreens! $49.99. It's as accurate as the Drs.
I brought it to an appt just to check it out. Its spot on.

I have around 10 different Oximeters and the two below are my favorites.
These will monitor your o2.

Wellue Bluetooth Pulse Oximeter
https://tinyl.co/4RfY

Vibeat Bluetooth Fingertip Pulse Oximeter
https://tinyl.co/4RfZ

Amazon.com: Wellue Bluetooth Pulse Oximeter Fingertip, Blood Oxygen Saturation Monitor with Free APP, Batteries, Carry Bag & Lanyard : Health & Household

https://tinyl.co/4RfY

Amazon.com: Vibeat Bluetooth Fingertip Pulse Oximeter with Pulse Rate, Blood Oxygen Saturation Monitor | Finger O2 Meter, Batteries and Lanyard Included, Free APP : Musical Instruments

https://tinyl.co/4RfZ

I asked a nurse why my reading always shows a good iso while I’m struggling to breathe. She said that it measures the oxygen level in your blood, not in your lungs.

Are you sure you're not having issues with cold fingers or poor circulation. That is the most common issue even for the doctors office. Mine is the same as my pulmonologist office uses.
I bought mine from the drug store and don't buy the cheapies online from Amazon. You get what you pay for.
The ones a your local pharmacy will be medical grade, and be within 3 points of accuracy. Keep in mind different fingers and hands will register different numbers and you body take and exchanges O2 for CO2 at different levels as needed.