This is always an interesting question that I have answered (I am a specialist in H2S Treatment). A higher alkalinity for MDEA stricktly means that the concentration of MDEA is higher. Since there is more MDEA present, this means that there is a higher concentration of MDEA, there will be more H2S that will selectively react with MDEA. This occurs up to a certain point. There MUST be free and available water in order for this H2S-MDEA reaction to take place. When the concentration of MDEA becomes too high, then there is not enough water to act as a medium for the reaction to take place, then your efficiency for sweetening decreases signifacently.
Pure MDEA has only one strength. Each supplier typically adds water so that the freezing point of MDEA is decreased (similiar to the effect of glycol and water). Check the suppliers concentrations, and you will see a difference in concentrations. If the concentrations are different, and it is MDEA, something 'else' is going on...
If you have further questions, feel free to reply.