No checkmating material left on the board still it was declared win to whom, has the Bishop,

Can Bishop checkmate alone?

Yes, it clearly can. The pawn can promote to a knight (or bishop), the black king can end up on a1, the black knight on a2, the white king on c2 and the white bishop on b2 and then it's checkmate.

But why declare it as winning for white when it happends once in a blue moon ?

"But why declare it as winning for white when it happends once in a blue moon ?"

That's why! (random example) How often do you think you see a blue moon here?

lichess.org/Nys8Nlbk/

Perhaps more to the point... here's a game Nordlandia won on time (although Black very likely will play Rxf5 after which it is difficult but possible to imagine how Black could lose):

The ICC server will immediately terminate a game in a draw if any of the
following positions occur: K-K, KN-K, KB-K, KN-KB, KN-KN, KB-KB. (K=King,
N=knight, B=bishop). Also, you cannot win on time if you have only a King,
a King and knight, or a King and bishop. (In the very rare event that you
had a forced mate in such a position, you can ask an administrator to
fix the rating result after the game.)

99% vs 1%

Well, I play FIDE otb as well as lichess and playchess. No issues.

The US with their home-made rules presented in USCF, ICC, chess.com... Play there or just shut up.

@ CM Sarg0n
what do you mean by "Play there or just shut up"

because in such situation you can always call the arbiteur OTB.

No. In rapid + blitz without increment you can‘t for example. @thecca

PS: I mean you can certainly, although in vain.

It is silly idiotic FIDE rules. And more: lichess adopted it PARTIALLY, that is, lichess engine do not declare draw in some dead draw positions (that will be draw according to FIDE OTB rules).

Several years ago lichess had sane rules, but then changed it without notifying the players.