Everything works fine now and the same values are saved in column and BLOB.
The problem does not occur anymore and everybody is happy. ;-)
BUT ... there is still an open question. Can anyone explain why the data was stored differently in the column and the BLOB before the fix? Ok, we handled the data the wrong way and the column was not properly set up, but usually I expect that the values in column and BLOB should be the same. So why were the decimal places saved in the BLOB?
***Edited by Moderator Marissa to update platform capability tags****
Probably different conversion code was used for each, with different results. You don't say how the column is/was defined, nor do you say how the property in the blob was stored. Once you clearly identified the value as a decimal (or double), both knew exactly what to do.
Originally the column was set to Decimal(18,0). To solve the problem we altered it to Decimal(20,2).
When I started to write this comment I wanted to recreate the 'problem' step by step to give you a detailed description. After creating a small data transform for test purposes I realized that there was a total misunderstanding on my side about the handling of the source string when it was passed into the target double. I thought the decimal places should have been lost in the double property. Shame on me - the decimal places were passed to the property and saved in the DB's BLOB. So, there was no miracle behind this behavior. Because of the false column settings it's totally clear that the saved value was shown there without the decimal places.