Update SQL has high Buffer Gets

I have a Update SQL which has three Columns in the WHERE Clause. There is a Index on these three Columes and Explain does show that the Index is used. Also, if I look in the SGA (with Toad SGA Trace) I see that the Index was used.

Looking into a AWR Report (and in Toad SGA Trace) I see that the SQL updates 1 Row and makes 105'000 Buffer Gets for each execution. This does match the number of Blocks in the Table pretty close.

The Number of Distinct Keys from the Index is almost the same as the Number of Rows in the Table.