Yes you’re wrong - you have no way of knowing what’s going on with that pH pen - just return as defective
Which model?
How is it calibrated?
You really need a 3 point calibration over the pH range you plan to measure AND an intermediate pH standard to test before determining that a pH probe is working as “promised”
Many cheap pH pens use an electronic single point calibration ...
- so put a point on some graph paper & draw a bunch of lines through it - any of those lines may be what your pH “pen” may be doing
- then put a point at 4 and a point at 6 and draw a straight line, then extend the line to 8 as a) a straight line, then b) as a plateau line - both of these line shapes are very common in experimental measurements
You’ve no way to knowing whIch line your pH pen is following or even if it is a smooth curve
The discrepancy between the pH pen & the test kit pH results from 2 different sources is not encouraging
A pH probe should be calibrated or calibration checked daily so last week’s calibration (which sounds questionable) is like saying it didn’t rain last Thursday, so it won’t rain this Thursday
Contact your water supplier for water analysis reports over the last 1-5 years, this will allow you to see trends & fluctuations in water quality