Eventtime changes number of cases

Hello !

I am trying to display the number of cases with a change activity on a monthly basis (ROUND_MONTH("_CEL_O2C_ACTIVITIES".“EVENTTIME”)).

Without this column I obtain the right numbers but as soon as I display the column, numbers increase.

To made sure I have a analysis filter on 2018-01. Without the month column I have:

When I put the column I have:

Does that come from the PU function that does not handle well event time?

The forumla for Change Rate Case is
COUNT(
CASE
WHEN PU_COUNT(“VBAP”,"_CEL_O2C_ACTIVITIES".“ACTIVITY_EN”,"_CEL_O2C_ACTIVITIES".“ACTIVITY_EN” IN (<%=changedFields%>)) > 0
THEN 1.0
ELSE NULL
END
)

Thank you in advance

EDIT

The problem comes from using “_CEL_O2C_ACTIVITIES”.“EVENTTIME”. Using the sales order creation date gives me what I want.