Understanding INNER JOIN logic

Question

I have the following table Exchange Rates Schema:

name	type	kind	null?	default	primary key	unique key
COUNTRY	VARCHAR(10)	COLUMN	Y		N	N
RATETYPE	VARCHAR(6)	COLUMN	Y		N	N
FROMCURRENCY	VARCHAR(3)	COLUMN	Y		N	N
TOCURRENCY	VARCHAR(3)	COLUMN	Y		N	N
STARTDATE	VARCHAR(12)	COLUMN	Y		N	N
RATE	NUMBER(15,7)	COLUMN	Y		N	N

Of which I only want the USD/MTHEND rows, i.e.:

SELECT FromCurrency, ToCurrency, Date(StartDate, 'YYYYMMDD') AS StartDate, Rate 
FROM EXCHANGERATES
WHERE DATE(StartDate, 'YYYYMMDD') > CURRENT_DATE - 15000 AND RATETYPE = 'MTHEND' AND ToCurrency = 'USD'
ORDER BY FromCurrency, ToCurrency, StartDate;

FROMCURRENCY	TOCURRENCY	STARTDATE	RATE
JPY	USD	2018-12-01	113.4700000
JPY	USD	2019-03-30	0.0090342
JPY	USD	2019-06-28	0.0092721
JPY	USD	2019-08-02	0.0093388
JPY	USD	2019-08-30	0.0093967
JPY	USD	2019-09-27	0.0092729
JPY	USD	2019-11-01	0.0092592
JPY	USD	2019-11-29	0.0091315
JPY	USD	2019-12-28	0.0091174
JPY	USD	2020-02-01	0.0091675
JPY	USD	2020-02-29	0.0091802
JPY	USD	2020-03-28	0.0092157
JPY	USD	2020-05-02	0.0093431
JPY	USD	2020-05-30	0.0093266
JPY	USD	2020-06-27	0.0093361
JPY	USD	2020-08-01	0.0095812
JPY	USD	2020-08-29	0.0094144
JPY	USD	2020-09-26	0.0094966
JPY	USD	2020-10-31	0.0095739
JPY	USD	2020-11-27	0.0096061
JPY	USD	2020-12-26	0.0096525
JPY	USD	2021-01-30	0.0095693
JPY	USD	2021-02-27	0.0094197
...	...	...	...
JPY	USD	2022-02-26	0.0086700

But there is no End Date column, hence I have the following query using self INNER JOIN to set the end date:

    SELECT
    EX.FromCurrency,
    EX.ToCurrency,
    DATE(EX.StartDate,'YYYYMMDD') AS StartDate, DATE(EX2.EndDate,'YYYYMMDD') AS EndDate,
    EX.Rate    
FROM
    EXCHANGERATES EX
INNER JOIN(
    SELECT
        FromCurrency,
        ToCurrency,
        Max(StartDate) AS StartDate,
        20251231 AS EndDate
    FROM
        EXCHANGERATES
    WHERE
        RateType = 'MTHEND'
    GROUP BY
        Fromcurrency,
        ToCurrency
UNION
    SELECT
        E2.FromCurrency,
        E2.ToCurrency,
        Max(E.StartDate) AS StartDate,
        to_number(to_char(DateAdd(DAY,-1,To_Date(to_char(E2.StartDate),'YYYYMMDD')),'YYYYMMDD')) AS EndDate
    FROM
        EXCHANGERATES E
    INNER JOIN 
EXCHANGERATES E2 ON
        E.StartDate < E2.StartDate
        AND E.RateType = E2.RateType
    WHERE
        E.RateType = 'MTHEND'
    GROUP BY
        E2.FromCurrency,
        E2.ToCurrency,
        E2.StartDate) AS EX2 ON
    EX.FromCurrency = EX2.FromCurrency
    AND EX.ToCurrency = EX2.ToCurrency
    AND EX.StartDate = EX2.StartDate
    AND EX.RateType = 'MTHEND'
WHERE
    Ex.tocurrency = 'USD'
ORDER BY    1,  2,  3;

FROMCURRENCY	TOCURRENCY	STARTDATE	ENDDATE	RATE
JPY	USD	2019-12-28	2020-01-31	0.0091174
JPY	USD	2020-05-02	2020-05-29	0.0093431
JPY	USD	2020-05-30	2020-06-26	0.0093266
JPY	USD	2020-06-27	2020-07-31	0.0093361
JPY	USD	2020-08-01	2020-08-28	0.0095812
JPY	USD	2020-09-26	2020-10-30	0.0094966
JPY	USD	2020-10-31	2020-11-26	0.0095739
JPY	USD	2020-12-26	2021-01-29	0.0096525
JPY	USD	2021-01-30	2021-02-26	0.0095693
JPY	USD	2021-02-27	2021-03-26	0.0094197

Why is the INNER result different to tinazmu's query using LEAD below? The below captures all unique USD/MTHEND rows with proper End Date:

SELECT
        FromCurrency,
        ToCurrency,
        DATE(StartDate,'YYYYMMDD') AS StartDate,
        LEAD(DateAdd(DAY, -1, Date(StartDate, 'YYYYMMDD')),1,'2025-12-31') 
            OVER (PARTITION BY FromCurrency, ToCurrency, RateType 
                    ORDER BY StartDate) as EndDate, 
        Rate    
FROM
    EXCHANGERATES
WHERE RateType = 'MTHEND' AND ToCurrency = 'USD'
ORDER BY FromCurrency, ToCurrency, StartDate;

FROMCURRENCY	TOCURRENCY	STARTDATE	ENDDATE	RATE
JPY	USD	2018-12-01	2019-03-29	113.4700000
JPY	USD	2019-03-30	2019-06-27	0.0090342
JPY	USD	2019-06-28	2019-08-01	0.0092721
JPY	USD	2019-08-02	2019-08-29	0.0093388
JPY	USD	2019-08-30	2019-09-26	0.0093967
JPY	USD	2019-09-27	2019-10-31	0.0092729
JPY	USD	2019-11-01	2019-11-28	0.0092592
JPY	USD	2019-11-29	2019-12-27	0.0091315
JPY	USD	2019-12-28	2020-01-31	0.0091174
JPY	USD	2020-02-01	2020-02-28	0.0091675