Writing 135 words and not 140 is not a big deal. However, writing 50 or 100 is. Examiners instinctively know – or they can see – when a text is too short.
If your text is much too short, you’re in trouble and you’ll probably receive a lower score in all four subscales (Content, Communicative Achievement, Organisation and Language).
If your text is too short, you can’t fully answer the question, so your Content mark will be lower. For Communicative Achievement, your ideas won’t be detailed enough and you won’t hold the reader’s attention. Writing a short text also means that you can’t demonstrate (1) how good you are at organising a text or (2) what a wide range of vocabulary and grammar you have.
So writing a short text really is bad news for your Writing exam result.