A rank checker asks for a result page from a chosen place and device and reports where your page appears. That sounds absolute and is not, because there is no single result page to check. What is shown varies by location, device, sign-in state and time, so two checkers can both be right and disagree, and a number without its conditions attached cannot be compared with anything.
Why two checkers disagree
They check from different places, with different settings, at different moments, and they treat the features on the page differently. If one counts a block of results as a position and another does not, their numbers differ by one before anything else varies. None of this makes either wrong; it makes them incomparable, which is why mixing sources in one series produces a jump on the month somebody switched.
Checked position against average position
Search Console reports the average position your pages actually appeared at, across real impressions in a period. A checker reports a single observation. The average is more representative of what people saw; the observation is more precise about a moment and can cover pages that are not yours. Use whichever suits the question and never put the two in the same column.
What to do with a number you cannot fully trust
Use it as a series rather than a fact. One reading is nearly meaningless; twelve readings taken the same way tell you a direction, and direction is what decisions are made from. That is also why consistency beats accuracy here: a slightly wrong number measured identically every month is more useful than a perfect number measured differently each time.
Recording the conditions
Source, device, location, date. Four fields, written once per reading, that make next month's comparison honest. This is the part every spreadsheet skips and every argument about rankings comes back to, and it is the reason a record is worth having at all.
Questions people ask about rank checkers
Why do rank checkers show different positions?
They check from different locations and devices at different moments, and they count result features differently. Both can be accurate and still disagree.
Which is more reliable, a checker or Search Console?
They answer different questions. Search Console averages what real searchers saw of your pages; a checker observes one result page at one moment and can see pages that are not yours.
How often should I check?
Consistently. Monthly or weekly on the same day, from the same source, is worth more than frequent checks from varying sources.
Does Longtailo include a rank checker?
No. It holds the readings you take, with their source, device, location and date, and shows the series.