Why the Bohra Hijri date is a day ahead

Short answer: the Dawoodi Bohra (Misri) calendar runs one whole day ahead of the tabular Islamic calendar. It is not a different algorithm — it is the same arithmetic with a +1 day offset, which is why it never drifts and never needs a sighting.

The three reckonings this app offers

Setting	Offset	Who uses it
Dawoodi Bohra (Misri)	+1 day	The default. The Fatimid/Misri calendar, fixed in advance.
Islamic Civil (tabular)	0	The plain tabular reckoning, the base everything else is measured from.
Islamic Astronomical (tabular)	−1 day	The tabular calendar shifted back one day.

All three are tabular: the month lengths are fixed by arithmetic, so any date in the past or future is known exactly. The difference between them is a whole number of days and nothing else.

Why no setting claims to be a sighting calendar

Umm al-Qura, and any calendar that begins a month on local moon sighting, cannot be tabulated — that is the point of it. A month starts when the crescent is seen, which is a fact about a particular evening and a particular sky, not a formula. So none of the three options above pretends to be one.

Instead there is a separate per-user day adjustment, which shifts your dates by a whole number of days on top of the calendar you picked. If your local sighting puts you a day off the tabular reckoning, that is the control to use. It stacks on top of the calendar offset rather than replacing it.

Why this matters for a prayer log

Prayer records are stored against the Gregorian date, and the Hijri date is computed for display. That means changing your calendar setting re-labels your history rather than moving it — nothing you have logged is lost or shifted, and switching back restores exactly what you saw before.