I think the Spanish is OK. This is apparently comparing sales figures in two different years. "Ventas mismas tiendas" must, I think, correspond to the English expression "same-store sales":

"DEFINITION of 'Same-Store Sales'
A statistic used in retail industry analysis that compares the sales of stores that have been open for at least one year. Same-store sales compare revenues earned by a retail chain's established outlets over a certain time period, such as a fiscal quarter or on a seasonal basis, for the current period and the same period in the past (usually the same period of the previous year.) Same-store sales allow investors to determine what portion of new sales has come from sales growth and what portion can be attributed to the opening of new stores. "
http://www.investopedia.com/terms/s/samestoresales.asp

And "a semanas iguales" simply means "over the same number of weeks", I think. I haven't found any other set phrase for this. The following shows, at least, that the expression I've suggested has been used in a financial report:

"WILLIAM BAIRD PLC
Interim Results for the six months to 30 June 2000
[...]
"Baird Brands total turnover was level with 1999 but at constant exchange rates increased by 1.7%. Excluding the Lowe Alpine acquisition, like-for-like turnover declined by 1.8% over the same number of weeks."
http://www.investegate.co.uk/article.aspx?id=200009290700307...

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Note added at 23 mins (2015-03-17 15:19:47 GMT)
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The point being, of course, that they're trying to compare like with like: same stores (excluding new stores) and same time period.

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Note added at 32 mins (2015-03-17 15:28:43 GMT)
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Or perhaps it means adjusted to the same/an equal number of weeks:

"The Company will report actual results and also will compare the same-store percentage changes for each period "as adjusted" to an equal number of weeks with the prior year, by excluding the current fiscal year's 53rd week of activity."
http://globenewswire.com/news-release/2013/02/07/521803/1002...

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Note added at 3 hrs (2015-03-17 18:14:16 GMT)
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As I've replied to bigedsenior, I really think "semanas iguales" could mean either the same weeks of the year (as Jorge has suggested above) or the same number of weeks. I don't see that you can rule either of these out. So maybe "over the same weeks" would be best; it suggests the former but could also cover the latter.