Dear Sid Ji,
A very good question and I personally appreciate your keen observation. The reason why you observed the lower portion of the inflorescence being broader that the apical region is because in Elusine indica the inflorescence is raceme (racemose inflorescence).
A racemose inflorescence has an active growing region at the tip of its main axis, and bears flowers along its length; the oldest flowers are found near the base or, in cases where the inflorescence is flattened, towards the outside. Hence the flowers at the base has already opened as they older hence look bit broader while the flowers at the apical region are young and still in a compact bud condition hence thinner.
Hope this answers your query. If you have anything more please feel free to ask.
Nivi ji and Tanay ji, thanks for the idenentification. But the spike is narrower at the top than in the bottom isn't it ?