It's quite true that both level and standard can be regarded as synonymous. However, there is indeed a subtle difference between them.
Suppose there is a competition where there are ten levels (ten rounds; you can participate in a round only if you qualify in the preceding one). Now, there can be different players in each level. However, the standard of the players in each level differ. Agree? As in, the quality of players in each level are different. In level 8, there can be player who has just luckily cleared the seventh level. Also, there can be another player who plays extraordinarily well. This means, the standard of this player is way better than the former.
So this is where the difference between level and standard lies.
However, based on context, you can use either of them for the same purpose.
As regards to you being rejected from job, I think your boss just wanted to tell you that you don't write good English (according to him). Simple.