Please look at the following

By March 1994, India signed the first BIPA with UK, which had first raised the demand for such an investment protection pact. Similar agreements were to follow soon the same year with Russia and later with Germany, Malaysia, Denmark and other countries. By 2010, India had signed up with 72 countries and enforced these agreements too and another 11 were signed but are yet to be enforced.

I don't understand why the writer has used two different tenses here after 'by'. In the first instance he used simple past and in the second instance he used past perfect. Is the use of the two tenses inter-changeable? If not, then what is the difference between using simple past and past perfect in the above? 

Also please tell me if my understanding of the above is correct or not

When we use simple past with 'by' as in----By March 1994, India signed----- It means it was signed before March 1994 and we are looking at it from the point of view of now, hence we are using the simple last tense.

But when I checked the date on which it had been signed, the date was 14th of March, 1994. So it means by using the simple past tense the writer is including March also, but 'by' means before. I don't get the usage here. Please help. 

On the contrary, when we use past perfect with 'by' as in----By March 1994, India signed----It also means that we signed it before March 1994 and we are looking at it from the point of view of march 1994, hence we have used past perfect.

Please tell me if my above understanding is right or not. If not, then please tell me when to use past perfect and simple past.

Thank you