Shares of Wal-Mart ( WMT) fell Monday, even though the company said same-store sales for March are tracking at the high end of the company's plan, helped by an increase in traffic and seasonal Easter sales.

The company said total U.S. same-store sales for the five-week period Feb. 28 through the April 2 are set to come in toward the high end of its 4% to 6% guidance. March sales will be released on April 8.

Among the strongest categories for the U.S. in the week ended March 19 were Easter seasonal items, food and electronics. The West and the Southeast were the strongest regions for the week, the company said. Higher traffic made up about two-thirds of the same-store sales increase.

Wal-Mart noted that Easter is one week earlier this year, though it still occurs in April. As a result, Wal-Mart said some sales will be moved from April into March.

Piper Jaffray analyst Jeffery Klinefelter noted that Wal-Mart's expectation for March same-store sales is slightly higher than the 5% to 7% range it has forecast in each of the past seven months. He attributed the increase to the six pre-Easter sale days in the April period, compared with 13 a year ago, meaning that more pre-Easter sale says fall in March this year, than they did last year.

Nevertheless, shares of the world's largest retailer by revenue were lately dropping 60 cents, or 1%, at $58. They are off their 52-week high of $61.31 reached March 4.

A Monday research note from Sanford C. Bernstein analyst Emme Kozloff suggested that higher gasoline prices could impact Wal-Mart's future same-store sales increases because lower-income consumers might cut back on discretionary spending.

"The recent rise in gasoline prices has created a controversy over whether tax refund benefits will be offset by higher costs at the pump, thereby creating a stall-out in comp-store sales growth at Wal-Mart, which draws more economically-sensitive customers than other retailers," Kozloff said, noting that gas prices have jumped 16% year to date.

The analyst believes though that tax refunds should "more than compensate" for the gas-price increase, which she thinks could cost families an additional $50 from March through May.

Looking specifically at April and May, however, Kozloff noted that gas-price increases are expected to be greater than during the same month last year, which she thinks could hurt Wal-Mart's same-stores sales by 30 to 74 basis points in April and by 48 to 100 basis points in May.

Overall, Kozloff expect same-store sales to be up 6% in the first quarter and 5% in the second quarter.