Nut and Saddle Placement
Unfortunately not every ukulele is mathematically built to ensure that the nut and saddle are the exact distant apart to ensure pitch consistency/correctness throughout the whole neck.
Fret placement
Furthermore, nor are frets always placed in accordance with the 'pythagorous ratio', required for ukuleles (and other fretted stringed instrument) to intonate correctly. This is why you must check a ukulele's intonation prior to purchasing a ukulele (if buying online ask if they can demonstrate the intonation over video call).
Precision matters
So many factors can affect ukulele intonation: incorrect bridge or fret placement are the most noticeable and virtually impossible to fix, whereas small intonation differences due to the re-entrant tuning of a ukulele (the high G-string), can be accounted for by compensating a saddle. Every detail matters... even a tenth of a millimetre variable is enough to have the ukulele sounding completely out of tune no matter how hard you try to tune it.
How to check a ukulele's intonation
Firstly, tune the ukulele to C, G, Am, F. Then have a play of the ukulele, strumming a range of different chords in different keys, keeping an ear out for anything funky! (C, G, Am, F --- G, D, Em, C ---E, B, C#m, A). From my experience the number one tell tail sign of poor intonation is when you notice one or two funky sounding chords (that always sound out of tune even when you're ukulele is in fact in tune).
Secondly, do some scales up the neck (if you can!), or ask your salesperson to do so (this is good for checking fret placement issues as well as making sure the intonation isn't shocking. Generally, the further you go up the neck, intonation issues are amplified.
Thirdly, always do a harmonic intonation check. To do this, follow the following steps: (1) hover your finger very gently over one of the (G, C, E or A) strings at the 12th fret (check all of them, one at a time), (2) pluck the string, (3) quickly pull your finger away (you should hear a bell-like chime), (4) while the note is still ringing press down on the 12th fret, (5) take notice of the pitch similarity/difference between the harmonic and the fretted note (they should be exactly the same), (6) repeat for the other strings.
Note: the pitch should not be audibly different (but be careful not to bend the string when you fret it, as this can give you a 'false reading').
If you find it difficult to hear the difference in pitch between the 12th fret harmonic and the fretted note at the 12th fret, you can always use a tuner. In this case, make sure the string for example is perfectly tuned (to G, C, E or A), check the harmonic pitch (which will also be G, C, E or A), and then fret the 12th fret and check that is no more than 1/8 of a semitone out. In my experience anything more than 1/8 of a semitone generally can't be compensated for and will leave you dissapointed in the long run. However, if there is a small pitch difference, don't fret, it can be fixed by compensating your saddle! To compemsate your saddle you will either need to 1) order a compensated saddle online, 2) take your ukulele to a local luthier, or 3) get a vice out nad some thin files and file the saddle as depicted in this image.
What about Pono's intonation?