Prior to any further analysis, take into account that GA does not offer exact numbers, but an approximation. Since their approach to measuring is consistent in a given timeframe, what you can best expect from the tool reports is to see trends in data. If trends are consistent with your backoffice data, then you have a deal.
Yes i agreed with you that Google doesn't presents exact number of data. But in January i had around 12000 goals complete in sql and Google showed 11500 - which is normal Feb - the difference increased to 1000 March - Going further it came to 2,500 April - Already by today it is 2,800
So if anyone has suggestion please let me know, where i should check the issue.