OnCalculate method returning different values in real time and historic

using System;
using cAlgo.API;
using cAlgo.API.Collections;
using cAlgo.API.Indicators;
using cAlgo.API.Internals;

namespace cAlgo
{
    [Indicator(AccessRights = AccessRights.None, IsOverlay = true)]
    public class OnCalculateIssue : Indicator
    {
       

        protected override void Initialize()
        {
           
        }

        public override void Calculate(int index)
        {
            if(index == 0) return;
            double val = Math.Abs(Bars.ClosePrices[index] - Bars.ClosePrices[index-1]);
            var txt = Chart.DrawText(index.ToString(), val.ToString("N2"), Bars.OpenTimes[index], Bars.LowPrices[index] - Symbol.PipValue*5, Color.Black);
            txt.HorizontalAlignment = HorizontalAlignment.Center;
            txt.VerticalAlignment = VerticalAlignment.Center;

        }
    }
}