describe the difficulty that you overcome, and why was it difficult to overcome?

You got 26 not 25 for this speaking.

There is no great difference on 26 to 27. and the scores on testbig are compared to other users who got 27. for example, for this user, most of her speakings are over 27:

http://testbig.com/users/xiaoyangyangyangyang

It is related to the overall English capacity.