I created a simple trigger to increment a numeric value on an object if a certain task is created.  

trigger UpdateSentMail_01 on Task (after insert){ 

// Tasks that meet criteria, and new tasks to create
Task[] qualifiedTasks = new Task[0], newTasks = new Task[0];
    
// Map of SD Members
   Map<Id,SD_Member__c> members = new Map<Id,SD_Member__c>();

// Find qualifying tasks
for(Task record:Trigger.new)
        if(Record.subject!=null && record.subject.contains('Print Sf Order Submitted') && record.status == 'Completed')
           qualifiedtasks.add(record);
    
    // Obtain member ID values
    for(Task record:qualifiedtasks)
        members.put(record.whatid,null);
   
    // If there are any membes to query, do so.
    if(!members.isempty())
        members.putall([select id, letter_counter__c from SD_Member__c where id in :members.keyset()]);

double i = 0;

     for(Task record:qualifiedtasks) {
// Get the current Letter Counter value
   i = members.get(record.whatid).letter_counter__c;
// Increase the Letter Counter by 1
    i=i+1;

        members.get(record.whatid).Send_Monthly_Letter__c=False;
    }
    
     update members.values();

    for(Task record:qualifiedtasks) {
        members.get(record.whatid).Send_Monthly_Letter__c=True;
        members.get(record.whatid).Make_1st_Follow_Up_Call__c=False;
        members.get(record.whatid).Make_2nd_Follow_Up_Call__c=False;
        members.get(record.whatid).Make_3rd_Follow_Up_Call__c=False;
        members.get(record.whatid).Letter_Counter__c=i;
    }
    
    update members.values();
}

 Before I entered the test tasks using excel connector here is what my test data looked like:

Now, after entering in my test tasks, each record should have increment by 1.  Instead I got this:

Since I didn't want to include all 700 test records, another block of records had their Letter_counter__c field set to all 3s and another group all 11.

Any ideas what is happening?