Pitch change in tab + narrow 16ths notes

[Metalhead]

Sometimes when I write 16th notes, the space between the two
"bars" (you know the bars on the staves of 16th notes?) gets narrow
(more than usual) and the note played is higher than it's supposed to.
This is more common on the higher notes, e.g. the 19th fret on the E-
string in standard tuning. Why does this happen? Is it a bug? I've
checked so it's not some bend I've forgot to release etc.

Thanks in advance!