mistake while calculating sum

Hi,

I am using Crystal Report 13 with visual basic 2010. I am using the following formula to calculate the sum

@headerResetMetal (in header section)

Global NumberVar x;

Global NumberVar metalValue;

WhilePrintingRecords;

metalValue:=x;

x:=0;

metalValue

@sumOfMetal (in detail section)

Global numberVar x;

WhilePrintingRecords;

x:=x+CDbl({tmpValReport.MVALUE});

@footerSumMetal (in page footer)

NumberVar x;

NumberVar metalValue;

WhilePrintingRecords;

x:=x+metalValue;

The above formula sometime give the correct result but sometime while calculating the sum it take the first record of next page. For example

In Page 1 we have

200

450

500

Sum:: 1490 but it should be 1150

In page 2 we have

opening c/f: 1490

340

450

Total: 2280 but it should be 1940

I stuck here did not get any clue on net why this is happening. Could any one please help me on this ?