LinPlug Spectral - Manual (Page 54)

If the Arpeggiator is triggered with an exact timing your wont notice this,
however, if the triggered Notes comes more than only a bit too early or late
it might result in a longer or shorter first note. More info in the following info
box:
Help, the first Arp note is always of arbitrary length !
The truth is, that its not arbitrary. There is a system behind it: When
you play a note, and the host transport is playing, then your note is
either exact in the timing or its more or less slightly before or after
the beat. Thats where the changes in first note length come from:
when you play the note too early, the Arpeggiator makes it just so
much longer that the following notes are in sync with the song again.
Same the other way around: when you play a bit late, the first note
will be a bit shorter to get back to sync.
Its a bit different when transport is stopped, then Spectral uses its
own internal sync, which resets after each pause of 1 second. So
after a pause it will start its internal sync with the first note, unless
your pause is shorter than a second, than it assumes you try to stay
in the timing you played before the pause.
To put it short: If you want all notes to be the same length, the
simples way is to have a tight timing for all notes you send to the
Spectral's Arpeggiator.