Account-level Twitter impressions on Iconosquare are not calculated in the same way as on Twitter's native analytics.
The Total Impressions KPI on Iconosquare (found on the Overview and Impressions sections) refers to the total number of views received on your Tweets/Replies posted during the date range you have selected. This does not take into account views on Tweets/Replies posted before the date range you have selected, even if the views are "happening" during the selected date range.
The Tweets/Replies I posted between 21st - 27th October have generated 1200 impressions.
A Tweet I posted on 20th October was viewed by someone today (27th October) but that view will not be included in the "Total impressions" KPI shown in the screenshot below, since the Tweet was posted prior to the selected date range.
On Twitter's native data, however, when looking at total impressions for the date range of 21st - 27th October, that view would be included in the account-level impressions data, since this includes all impressions received during a selected date period, regardless of when the Tweet was posted.