Yes, either the slider on the Transport isn't exactly in the middle (see screenshot) or you haves an invisible metronome marking.
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Re: making Metronome Marks the Sib Reference file says:
Metronome marks look something like q = 72, and are often accompanied by a tempo marking (see Tempo below).
To write a metronome mark:
Select an object (e.g. a note or rest) in your score where you want the metronome mark to go, normally the first note of a bar, and choose Metronome mark from the Common category in the Text > Styles > Style gallery: a flashing caret appears. (If nothing is selected in your score, choose Metronome mark, then click in your score to place the text.)
Typography of tempo and metronome text Sudden changes of tempo should begin with a capital letter to startle you:
Molto vivace, Tempo I, Più mosso.
Gradual changes of tempo begin with a lower-case letter: poco rit., accel.
If tempo text or a metronome mark is above a time signature, the left-hand sides of both should be aligned.
Write notes in metronome marks by right-clicking (Windows) or Control-clicking (Mac) on the flashing caret to get the word menu. You can alternatively use the Ctrl or X key in conjunction with the numeric keypad.
You can then type = 60 or whatever in the normal way. Hit space on either side of the = sign.
When typing a metronome mark after a tempo mark, e.g. Allegro q = 60, you are not advised to input it using two separate bits of text (Tempo text plus Metronome mark text), or the two separate text objects could collide if the notes in the bar get too close together.