Unaccountably

She had known that the sight of Matthew jaunting off so unaccountably would be too much for her neighbor's curiosity.

After all that hard work, he unaccountably failed to take out the patent.

The jury was unaccountably slow to reach a verdict. He was clearly innocent.

The number 392, very unaccountably, was never filled, and two of the sets of parts never appear to have been assembled at all, and were presumably kept as spares.