This is a very good question. As we read the quran many times and find that the scripture is in some cases in total contradiction to the "school" Arabic we learned.
First you should know that this is how the sahaba (peace be upon them) who wrote it down when the quran was collected fixed it in the so called rasm al 'othmany (the scripture which have been fixed as amushaf at the time of the caliph 'Othman ibn 'Affan -3rd caliph). To be more exact, the sahaba have written the word الصلاة this way الصلوة so the pending alif ألف is a later addition (for الضبط which means and is used for correction or apprehension).
Note that the actually known Arabic spelling which we may call modern Arabic has been developed during the Omayyad Dynasty.
Later scholars have been in consensus that the quran should only be copied in the way the sahaba (May Allah be pleased with them) wrote it. For example Imam Malik who lived during the last days of the Omayyad dynasty and the the first days of the 'Abbasid dynasty has been asked whether it is allowed to write a mushaf according the "approved spelling" rather then the spelling the sahaba (May Allah be pleased with them) and answered: I don't agree to this, and my opinion is that he should write in the original scripture. This fatwa was quoted in al-Muqni' of Imam a Dani (a major source for rasm and qiraat and quran sciences) and he added and nobody among the scholars said otherwise.
Please note that the quran has in first place been transmitted through the time by oral memorization and recitation on a hand of Scholars beginning with the Sahaba (May Allah be pleased with them) and not through a book (even today)!
According to this fatwa scholars say that these differences between quran scripture and "modern Arabic" are a mystery or a part of the inimitability of the quran. So a clear and definite answer for your question can't be given.
But possible answers could be given (most from a-Dani' muqni'):
- one for our example is that some qiraat (readings) may read the word (the) prayer الصلاة in singular while others may read it in plural الصلوات so the spelling of the sahaba الصلوة may indicate both possibilities in one word!
- An other possible explanation is that the origin of the alif of الصلاة was a waw واو،
- A 3rd possible explanation is that the alif has been emphasized and when the Arabs emphasize an alif the pronunciation tends to a waw so it is not pronounced as-salaata but as-sala(w)ta (well it's difficult to transliterate, but I hope you can get the meaning if you listened to a recitation, I'd recommend a recitation of qiraat warsh 'an Nafi').
- A 4th explanation is the importance or meaningfulness of these words, so they should have been emphasized in that way.
- And in المنهاج شرح مسلم بن الحجاج للنووى the commentary of Imam an-Nawawi on Sahih Muslim we find fifth explanation: they have been written in the language of al-hyraلغة الحيرة whom have thaught the Arabs the scriptur (handwriting).
Other examples for differences between rasm and modern Arabic where a waw replaces an alif:
الزكوة ـ الزكاة (zakat)
الحيوة ـ الحياة (hayat:life)
الربو ـ الربا (riba)
بالغدوة ـ بالغداة
كمشكوة ـ كمشكاة
النجوةِ ـ النجاة
ومنوةَ ـ ومناة
Words that have been written with a ta' or ha' or ha' instead of a ta' at t'anyt:
رحمت ـ رحمة ـ رحمه
(see also نعمة ـ لعنة ـ امرأة ـ شجرة ـ سنة ـ قرة ـ جنة ـ فطرت ـ بقيت ـ ابنة ـ كلمة)
Words that have been written with two ya's instead of one like:
بأييكم ـ بأييام ـ فبأيى ـ بأييد ـ نبأىْ
My reference in Arabic.
And Allah knows best!