FutureBuilder gets 31 Elements from Future, but its snapshot has 129 Elements - flutter

Question

I have a Future<List> function that fetches data from an API and it returns 31 list elements for the month july.
However, when I plug this Future function into a futurebuilder, the list length is 129 elements. Code is below. What could happen here that doubles the elements?
My Future code:
Future<List<DataDay>> getMonthlyDataHighlightDays() async {
String url = "myUrl";
var response = await http.get(Uri.parse(url));
Map<String, dynamic> DataResponse = jsonDecode(response.body) as Map<String, dynamic>;
List<dynamic> monthlyList = DataResponse["day_predictions"];
List<DataDay> monthlyListFixed = [];
monthlyList.forEach((dayElement) {
DataDay dayData = DataDay.fromJson(dayElement);
dayData.hour_predictions.forEach((hourElement) {
if (hourElement.rainbow || hourElement.pattern.toLowerCase().contains("rain")) {
monthlyListFixed.add(DataDay.fromJson(dayElement));
}
});
});
log("MonthlyListLenght: " + monthlyList.length.toString(), name: "getMonthlyDataHighlightDays()");
return monthlyListFixed;
}
This is my Futurebuilder:
Future<List<DataDay>> monthData = getMonthlyDataHighlightDays();
Display(
title: "Highlights this Month",
children: [
FutureBuilder<List<DataDay>>(
future: monthData, // a previously-obtained Future<String> or null
builder: (BuildContext context, AsyncSnapshot<List<DataDay>> snapshot) {
Widget child;
log("Snapshotlength: " + snapshot.data!.length.toString(), name: "FutureBuilder<List<DataDay>>");
if (snapshot.connectionState == ConnectionState.done && snapshot.hasData) {
List<DataDay> monthlyList = snapshot.data!;
log(monthlyList.length.toString(),name: "DataPage");
child = ListView.builder(
shrinkWrap: true,
physics: NeverScrollableScrollPhysics(),
itemCount: monthlyList.length,
itemBuilder: (BuildContext context, int dayHourIndex) {
DataDay dayData = monthlyList[dayHourIndex];
DateTime date = DateTime(dayData.year, dayData.month, dayData.day);
return Row(
mainAxisSize: MainAxisSize.min,
children: [
Text(printDateTimeToStringWithoutYear(date, settings)),
SizedBox(
height: 10,
child: ListView.builder(
physics: NeverScrollableScrollPhysics(),
scrollDirection: Axis.horizontal,
shrinkWrap: true,
itemCount: dayData.hour_predictions.length,
itemBuilder: (BuildContext context, int hourIndex) {
return getDataIcon(dayData.hour_predictions[hourIndex].pattern, width: 10);
},
),
),
],
);
},
);
} else if (snapshot.hasError) {
child = Row(
children: [
const Icon(
Icons.error_outline,
color: Colors.red,
size: 60,
),
Text(snapshot.error.toString())
],
);
} else {
child = Row(children: [
SizedBox(
width: 60,
height: 60,
child: CircularProgressIndicator(),
),
Padding(
padding: EdgeInsets.only(top: 16),
child: Text('Awaiting result...'),
)
]);
}
return Center(
child: child,
);
})
],
)